They will almost certainly go back to the coop when it starts to get dark, probably to the coop they are used to, at least at first. Once they really integrate and get to know each other, they may all go to the same coop. But if they have been in there long enough to know it as home, they will go back to the coop.
The only problem I have had with this, and it seems to happen to a few each time, is that the first time out, they get stuck on the wrong side of the fence. They want desperately to get to the coop, but they forget where the gate is. I have to sort of herd them around the fence until they get to the gate, then they go right in. With a few, I've had to do this two nights straight before they learn where the gate was. I had one Buff Orp pullet that looked back over her shoulder at me on the second night I did that for her, as if to say, "What took you so long? I was getting worried"
I don't think you can let them out at the same time without the roosters fighting, if they are mature roosters. If one is mature and one is still immature, the dominance issue may involve more chasing than fighting, but at some point they have to determine which is the dominant rooster. They may reach an accommodatiion where they agree which is dominant and which is not dominant and form a good partnership in taking care of the flock. Or one may kill the other. I can't tell you what will happen.
Your best bet may be to alternate free ranging, one group one day and the other group the next.
