I fail to see how there's an "if" involved. The two teams are already explicitly connected by Thanos and the infinity stones. Even if it turns out not to happen in Avengers 3, there is exactly zero possibility that they won't meet up at some point.
Johnny Novgorod said:
Wouldn't that make too many characters for one movie?
Not necessarily. My biggest problem with Guardians was that there were too many characters, but that's mainly because they were all new (as far as movie audiences are concerned at least) and there simply wasn't time to introduce them
and give them all a decent amount of screen time and development in a single film. But by the time any crossover could happen, every one of the Avengers will have appeared in at least three previous films, with most having had at least two or three solo films on top of the minimum of two Avengers films they'll have had (Hulk being the only one with just one film). Assuming Guardians 2 isn't the crossover point, the Guardians will have had that film to flesh them out a bit more.
Sure, a crossover would have an awful lot of characters compared to most films, especially if they include many of the secondary characters as well, but they'll all be familiar enough by that point to allow a film to focus on what they're doing rather than having to spend all its time reminding us who everyone is and why they're there.