Aside from Crisis on Two Equestrias and Teatime for Two, I have yet to see that many other crossover's with the Celestiaverse Six meeting the Lunaverse Six. As such, I have an idea for one that I which to share with anyone that is interested:

Title: Through the looking glass.

Main characters: The Main Six, The Lunaverse Six, Princess Celestia, Princess Luna, Nightmare Moon, Zecora & Spike.

Genre: Alternate Universe, Comedy, Dark, & Adventure.

Premise:

Cannon Placement:
Lunaverse: After Longest Night, Longest Day.
Celestiaverse: Right before the first episode.

Following a strange pulse of magic, Trixie find's an interesting mirror in the Castle Ruin's of Everfree Forest attempt's to bring it back home to study it. However upon levitating it with her magic, she is given a vision of Princess Luna becoming Nightmare Moon and Tyrant Sun somehow rising to power. Believing it to be a vision the future she quickly brings it back to her home, unaware that a certain Dragon and Zebra were watching her...

Trixie gathers the other Element wielder's as well as Princess Luna herself who explains that the mirror is an ancient portal that she discovered sometime after Celestia was banished, and what she saw was in fact a vision of an alternate Equestria. But before she can explain further, Zecora appears, using several plunder seeds to over take the group as Celestia briefly subdues Luna. She reveals her intent to take over the other Equestria by killing original ruler and posing as the ruler, however as she attempt's to go through the portal with Zecora and Spike. Trixie break's free first and attempt's to use the power of the Element's to stop her as she enters, only to end up causing a chain reaction.

The mirror emits a powerful wave of magic that envelops everyone in the room before sucking everyone into the portal, as they are flung into the alternate Equestria, the Lunaverse six find themselves merged with their own Element's before they are separated and scattered across Equestria. While Luna's end's up in Canterlot...

Meanwhile in another part of the Everfree Forest, Celestia (Now back to calling herself Tyrant Sun) encounter's Nightmare Moon. Upon comparing notes on their own reality's, the two decide to work together in order to dethrone their counterpart's sister's (both intending to back-stab each other when the time is right..).

Major note: The Lunaverse six's spirit's have been trapped in their Element's, when worn their spirit's take control over the wearer. They can interact with the wearer's through their mind.