Honestly I don't see that happening and all. If Marvel has decided to finish off this whole arc using two films instead of one, I honestly don't see it being a problem for most if not all of their actors and contracts. Call me crazy but I don't think these guys are THAT eager to jump ship, and I think they can work out an agreeable rate at which to extend the contract or come back for one last film.
Yeah they're actors and all, but none of them seem like they hate making these movies THAT much, and they're not having to commit that much more time if both films will be shot within a reasonable time frame of one another.