Since the new update, I haven't been able to get two monsters when I join someone's battle or someone joins mine in the beginning areas, since the beginning of the game, Wizard City, and it's quests were changed around. We only get two enemies when a 3rd person joins. This happens all of the time on Unicorn Way and occasionally on the three streets (Cyclops Lane, Triton Ave, & FIrecat Alley). I also have an issue sometimes getting two enemies by myself in Colossus Boulevard.
It seems like this is just a balancing thing, yes this seems to happen from time to time for whatever reason. Occasionally on some of the early streets, I have trouble even starting a battle to begin with. This does go away as you get further on in terms of what areas you're battling on though.
Yeah, now that I'm in Krok on my lowest character, I'm not seeing this problem at all anymore. Besides the intentional (I think) change where people can't join your fight if you're finishing off the mobs that turn.