I like to imagine Marceline and Bonnie met somehow like this:
-Marcy passes out underneath the treehouse (not grown that much) due to not drinking enough blood, Schwabl with its animals instinct runs out to find help. PB is doing research outside the kingdom with PepBut, Schwabl bites on PepBut running away with him in Marcie's direction & PB is forced to chase after them. PB sees Marcy takes her to the candy kingdom.-
I also like to think that PB found out that Marcy can drink red because in her diary song she said "I only see red, blood, skys" so I'm guessing that when she gets hungry she sees blood veins or whatever and shades of red, & PB figured that out, & now Marcy can eat without hurting anyone.
That's all I've imagined about their past together. ; )