Well I was talking to t2wave about my own concept for a Marvel/DC crossover. I've done it on here before but the idea is essentially combining the Young Avengers and Young Justice into a single team.  I never really put any reason into how the two universes are together because to me thats over thinking things.  You can make a universe where both DC and Marvel heroes co-exist without having to stuff a reason into it.  Simple, and to the point of the matter.  

Really also when you think about it whats the possibility that the thing that brings everything together really plays a serious role in what happens.  Only the story is like a reality like ours suddenly has DC and Marvel heroes and stuff in it.