Assume I have two Heroes (Hulk and Iron Man) in ready status and I move them to an adventure with the Nemesis participating in it. I first have a fight with Hulk against Rhino. During the fight I get 2 K.O., but I win in the end. Now I want to confront my Nemesis "Red Skull". Do I now have to fight with Hulk again, because I used him in the fight with the normal villain before, or could I now choose to use Iron Man instead, because he is also active in this area?
When you have two heroes in the same area and you are attempting a mission, you can choose which one you want to use in combat at the beginning of each single round of combat. This means that you can alternate them as you see fit, using one during the first combat sequence, then possibly using another, and so on.
Hope this helps,