Did you also do a thorough house cleaning? If they had mites (and it sounds like they might have) then their house needs to be cleaned from top to bottom, too. Mites in large numbers are so devastating to chickens that you may lose a few more birds from the original infestation. They can't build up their blood supply overnight, so if they were compromised to the point of near death then it isn't going to take much to kill them off over the next few weeks. You are going to have to keep your birds in the optimal conditions to make sure they survive the next couple weeks. They will bounce back, but it will take some time.
Good luck. I'd also check the other house very carefully for an infestation as well. It's unusual that two houses in close proximity would have only one affected.
