One day a pair of Potara earrings appeared in the human world, one of them appeared in Sunset Shimmer's room at her house and the other appeared in Rarity's room at her house.
Rarity put one of the ear rings on her left ear while Sunset Shimmer put the other ear ring on her right ear.
Mere moments after doing this they were pulled together out of their houses towards each other, when the bodies of Rarity and Sunset Shimmer collided with each other they wound up fusing with each other, now the two people or in this case one person and a former pony turned into a person became one person.
When this new being looked into a mirror she noticed that she had Rarity's blue eyes, half of her hair was Rarity's indigo hair while the other half was Sunset's red hair with yellow highlights in it, she had half of Rarity's gray shirt while the other half was Sunset's magenta shirt with a picture of her pony version's cutie mark on it, she also had half of Rarity's purple mini skirt with three blue diamonds on it that looks very much like her pony version's cutie mark back in Equestria even though this isn't the same Rarity from Equestria however the other half of her mini skirt was Sunset Shimmer's orange mini skirt with white, pink and purple streaks on the side however she also had Rarity's purple boots as well.
Upon seeing this the brand new being screamed in horror not because two people have become one person but because both her shirt as well as her mini skirt was completely mismatched in terms of matching colors.
This new person called Sweetie Belle in the room "Sweetie Belle darling could you please come in the room please", however after she got done speaking she realized that it sounded like Rarity and Sunset were talking at the same time."
As she was coming upstairs Sweetie Belle asked "Why does it sound like you and Sunset are talking at the same time?", but stood there completely silent at this brand new person who was standing right before her.
After several minutes of silence Sweetie Belle asked "What's going on here, who are you anyway are you my sister Rarity or Sunset Shimmer, I can't tell?"
The new person who hasn't decided on a name for herself yet told Sweetie Belle "That's just it I can't tell either, I was going to write Princess Twilight about this to ask her what might have happened but not even I know, oh Sweetie Belle please tell me what to do and also what to call myself."
Sweetie Belle thought about it for several minutes and said "Hmm well half of you is dressed like Rarity and even has Rarity's hair but the other half of you has Rarity's hair and is dressed like Sunset, so how about until we figured out what happened let's call you Sunsarity."
Sunsarity told Sweetie Belle "Sunsarity huh, that does have a nice little ring to it doesn't it, ok Sunsarity it is, but just one more thing."
Sweetie Belle asked Sunsarity "What is it Sunsarity?"
Sunsarity asked her sister "I'm still going to ask Princess Twilight for help and everything but how am I going to explain this to our parents and even more importantly how can I possibly go to school like this?"
Sweetie Belle then told Sunsarity "I guess I'll just tell mom and dad that you're sick today and then they'll call the school and tell them the same thing."
Sunsarity then asked Sweetie Belle "Ok but what if they come and check on me, I mean I don't know how I'm going to explain this to them?"
Sweetie Belle thought about it and told Sunsarity "Oh I know, I'll tell them that you caught a illness from Equestria and only Princess Twilight can come here to heal you."
Sunsarity said as she went over to hug Sweetie Belle "Why Sweetie Belle darling that's brilliant, It's too bad that I can't tell if we're still related while I'm like this."
Sweetie Belle told Sunsarity "I'm sure we still are I mean after all you're half Rarity right so I guess this makes you my half sister."
Sunsarity then told Sweetie Belle "I just hope that Princess Twilight can find a way to separate us, so that I don't have to stay this way."
Sweetie Belle told Sunsarity "I'm sure she will, bye Sunsarity, I'm off to school now."
Sunsarity was about to write in Sunset Shimmer's magic book in order to write to Princess Twilight back in Equestria but then she realized "Oh no Sunset's book, I mean my book is back at Sunset's house, I guess I'll just have to sneak out of here to get it, It's a good thing Sweetie Belle is covering for me, I just hope that noone else see's me while I'm doing this."
Sunsarity managed to do this without getting caught or noticed and snuck back into her room back at Rarity's house.
Sunsarity then wrote to Princess Twilight as she talked outloud what she wrote "Dear Princess Twilight I know this will sound strange but this isn't really Sunset Shimmer writing to you nor it is really Rarity, you see somehow two ear rings appeared in our houses and when we put them on somehow we both became one person, Sweetie Belle is covering for me, I told her to cover for me by telling our parents that I'm sick and that I've caught a illness from Equestria that only you can cure, of course this is not the case but I really don't want anyone to see me like this, but anyway can you please come over here to find a way to separate back into two separate people,
Please come and help signed Sunsarity.