The Simon/Marcy connection seems to be one of the pinnacle parts of the show to be honest. Even those that don't generally care for the random humor or over the top characters often tend to at least find themselves getting a little interested when the little half demon girl and tortured antiquarian struggling to get by in a dying world come on screen.
trojan-rabbit seems to have the uncanny ability with her writing to manage to stay in character with the show while still managing to try her best to break the hearts of her readers into pieces tiny enough that you could fit them through the eye of a needle ;_;
Not that that's stopped me from reading pretty much everything she's written mind you.
Aghrghgh, hurk! Why?! Why do you do this to my feels? It must have been awful for them both. Simon knew he was slipping into madness, and he was helpless to stop it. Marcy, on the other hand, could do nothing but watch and try to keep him with it for as long as possible. Geez, Adventure Time has some seriously dark and twisted history.
From the show, we don't know much about how long they were together, when they left each other, etc. I actually wrote a fic a while back about Simon leaving (here), but I don't think it fits in with the canon.