You can do a lot to increase the efficiency even with one AC by making sure the plenum area is taped up correctly and that all of the ducts are sealed where they are cut out for the ceiling registers to insure that you are not blowing air into the ceiling area. I almost doubled the felt airflow of mine with a roll of duct tape. manufacturers typically do a pretty sloppy job of AC installation.
Solar screens are also a possibility and there are some pretty good products out there. I would recommend solar screen material over film simply because with film, the heat still makes it to the glass.
