Maybe I'm stupid, but even in context of the story, I have no idea what they mean by RomeoJulietting. All I can think of is suicide which it definitely isn't.
I found this:
In the show:
Romeo and Julliet reference compares the star-crossed nature that a romance between Suletta and Elan would be. Elan being from a different 'house' and Suletta being from Mercury. The idea here is that Miorine believes that Suletta is romantically pursuing Elan in episode 5. By going 'all romeo and juilet' on her she would be ditching her responsibilities (as Miorine's groom) to pursue her Romeo (Elan) in an act of love.
In this doujin:
In the doujin's case Suletta is referring to the whole tragic ending aspect of it. The fact that fate tears the star-crossed lovers apart - sort of deal. The 'RomeoJuliet' she's referring to is a turbulent end of their relationship through bitter means.
When she uses it after the kiss she's referring to how much closer they've gotten and how it wasn't supposed to be that they actually developed feelings for one another.
It's a flexible analogy using Romeo and Juliet. Same material referenced, but different meanings.