I'd say move everything to one home. Trust me, going back and forth twice a day (no matter how close) will get old before the first week is up! I babysat my friends cats before and by day three I was not thrilled to be running over to the other house to watch the kitties when all I wanted to do was relax and chill out for the night.

You mention AC... I remember when I babysat my brother's cat. He always kept his apartment at freezing. I was used to my dad keeping the house at 70 or above. I LOVED going over to babysit that cat because of how cool it was in there. Ahhh... how I remember watchign a movie with a quilt drapped over me and a purring cat in my lap in the middle of our worst heat wave Even with the luxury of awesome AC... I was still "over" the excitement by midweek