It is not often, almost never in fact, that ponies think of the death of their beloved diarchs but when they do they think only of the grief for those we loved them and their plans. For example, there is a night creature, a vampiric unicorn, who mastered stealth magicks and it's use in assassinations, despite Equestria being a place with very few contracts. This unicorn, whoever it was, managed to sneak into the castle entirely undetected and kill the sleeping sister, who was Celestia at the time since her first attack was in the middle of the night. In the same night but now near the end of it, with a creeping twilight that would last weeks, she would follow the tired and sleepy night time princess to her room, awaited her to sleep, then killed and dismembered her as well. Because of this and the real military threat Equestria would soon face, every guard in Canterlot and for that matter in all of Equestria would stay on high alert for the next several years
It did not take long for ponies to notice in the castle, then Canterlot knew, then Equestria knew, then the world knew of the death of the sisters. The unicorn escaped undetected from the assassinations. Twilight sent a letter immediately to the Crystal Empire upon hearing of her mentor's death before boarding a train to Canterlot. She would arrive there and await them several days before Cadenza and Shining Armour would show up. Their arrival from the train was greeted with much more ceremony, as the ponies of Canterlot with few exceptions had assumed Cadenza would take the place of the diarchs, in combination with Prince Blueblood, for however long he was around. These plans were of course shunted by the combined decision of Cadence and Twilight, Blueblood would be getting nothing.
While the new diarchs were racing to meet each other a threat to their reign had already organized itself, and the Empire of the Night was very intent on stopping Sparkle from being a ruler of Equestria. These creatures of the night, vampires, werewolf ponies, were afraid of a continued persecution of their kind under Equestrian law because of Twilight, whom they figured would want to keep everything Celestia did and was doing in place. The headquarters of this rebellious empire would be just on the other side of the Canterlot Valley, with just mountains blocking the path between them and Canterlot.
These night creatures took roughly two days to gather and organize into a collective, a new country if you will. Led by one Shallow Waters, a thousand year old vampire with a particularly bitter grudge against the old order of Equestria. The train from the Crystal Empire pulls into the Canterlonian station, essentially empty of life aside from the conductor and the royal family of the empire. Cadenza steps out first, waving half heartedly to the crowd that had gathered to watch her move from the station to the castle. It was a somber ceremony, everypony heavy with grief and uncertainty. For many ponies in Canterlot, the well off or business owning, this crisis would mean collapse and uncertainty in the markets for no pony knows how long, not to mention what treaties might be broken by foreign powers who had treated with the princesses, and not necessarily Equestria. The two are oft spoken of interchangeably, and treaties were made with the princesses in their name rather than in the name of Equestria. Their assumption of course was that they would continue ruling for a much longer time.
So Cadenza and Shining Armour, carrying Flurry Heart between them, quietly move to meet with Twilight in the castle. They each wave a few times to the crowd but otherwise did not interact very much with them. Upon reaching the castle doors, the two guards there, also set upon by sadness and weariness, would open them instantly. The two royals find Twilight sitting upon the Celestial throne, the empty Lunar one next to her still, though when she sees them come into the throne room Twilight Sparkle stands and rushes up to hug both of the ponies "I'm so glad you made it here unharmed!"
The two hug her back before Shining Armour says, half-joking and with a small attempt at a laugh which nopony joins in "Us too, Twily, it was a very dangerous train ride..."
"I think it would be great to spend some time joking around, but I fear there are some pressing matters we need to attend to first." Cadence says after clearing her throat for attention "For one, the sun and moon need to begin moving again. The gryphon and zebra lands must be cooking right now!"
"Well, for that I thought of an easy solution!" Twilight says, rather excited at having the answer to such an important question "We give groups of unicorns the job of moving the sun or moon, maybe a few groups of each!"
"Or, just one each." Cadenza smiles softly at her friend's excitement, though it is a sad smile "But who knows how to move the sun and the moon? There isn't exactly a book on the matter, all we know is that unicorns allegedly did it before Discord showed up."
Twilight sighs out in thought, taking a few moments before she just says "Well it can't be too difficult, if the first unicorns figured it out then so can we!"
Cadence and Shining nod in agreement, and afterwards the princess looks to her captain of the guard and says "Find a bedroom for us and Flurry, and get her to sleep. She must be tired."
Shining Armour becomes the only one holding Flurry Heart in his magic as he answers "Of course dear." Then walks out of the throne room and begins to walk about the castle. He knew it well from the time he spent running the Canterlot guard under Celestia, before he and Cadenza had married. Thusly he doesn't take a long time to find his ideal room, and he was certain Cadence would love it too. A few rooms down from the sun princess's bedroom is a large room roughly the size of the royal bedroom, but decorated rather heavily in various shades of purple, with pink and sky blue laced into the purple in long lines and swirls on the bedsheet. He sets the foal-carrier down on the floor as he floats Flurry Heart out of it and onto the soft bed. A cradle could be created later today, likely by him, he thinks as he does his best to settler her down and get her to sleep. This turns out easy enough for him, as the foal falls asleep within minutes
In the throne room, after Shining leaves, the two princess mares walk themselves to the thrones as they speak to each other on the sun and moon problem. When they reach the throne, Prince Blueblood steps into the doorway of the throne room and looks at the two new rulers on the throne. He is sad, like the rest of the ponies, he did lose family after all in the attack. That, however, didn't stop him from challenging Twilight's right to rule on her first day here, and now he was here again to try asserting himself to, to him, the highest position possible in the world. His appearance in the doorway stops both princesses in their discussion, and his approach towards the throne creates a gentle apology and question from Cadence and a suspicious gaze from Twilight "We're all very sad about your aunt and her sister's death, she was as close to everypony as she was to you. But I have to ask, what are you doing in the throne room?"
Blueblood comes close to the thrones before he says "Auntie would have wanted me to help rule Equestria, and I came here to ask the two of you for an equal seat next to the two of you."
Twilight speaks up before Cadence can "Well traditionally, Equestria has only two rulers, but we can have you be on the board of advisories."
He huffs lightly and says "I'm already on that, I've been helping auntie in that way since I was ten."
Twilight frowns a little "We don't want to change too much right away, but perhaps later on.. We could use some help figuring out how we're going to get the sun and moon moving again."
At that he gives a short "Ha!" Before saying confidently "Well the rulers are to move them, of course. One each, so- er," He stops a moment then says "That would mean you two, I suppose."
"But how are we supposed to do that?" Cadence asks "Our magic isn't as powerful as theirs."
"But you are alicorns."
The princesses roll their eyes at him and Twilight says "They had a special ability to control the sun and moon. Me and Cadence might have powerful magic, but it isn't meant for moving the sun and moon."
Blueblood begins to look concerned for the first time, and the three of them continue to discuss this topic until it was thought of and agreed upon that there would need to be a group of unicorns combining their magic with a princess to move either the sun or moon, until something better could be thought of. He leaves them to talk about other things and goes off to the garden, to admire the flowers. He would later leave Canterlot for reasons unknown to the rest of the city.
Throughout Equestria's history, ponies have advocated for a republican system but this faction of republicans has always been very small and dismissed due to the benevolence of the eternals they had before. Now that those two are gone these voices are louder, and no longer are dismissed as easily, as well some carrying independence sentiments. Ponies began to think more on politics, as others still insisted on the benefits of a continued diarchy but with two ponies chosen from the masses rather than purely alicorns or by blood.
This is all not to mention the anti-equestrian sentiment brewing in the zebra and gryphon lands. For nearly 100 hours they have been baking in the sun, and all knew that it was the two horse sisters that controlled where the sun and moon went. All three envoys sent to Equestria from the gryphon empire never returned there or even reached their destination to tell of the zebra and gryphon plight, due to a storm for one, the second ran into some wild, angry, and vicious ponies who managed to bring him down, and the third simply disappeared somewhere above the clouds, never to be seen again by pony or gryphon. By the 90th hour the empires of the gryphons were ready to go over to Equestria and either force the sun controllers to begin moving it again, or simply take over that country.