Thanks kwangtzu, I forgot to mention the shelf will have two 2x4s going front to back, like joists in a house, for support.
The two 55G are plexi so I don't worry about a small amount of flex. When I first got back into fish keeping, a trip to the LFS convinced me plexi was tuff stuff. All of their saltwater tanks were 8 to 12 inches LONGER that the supports. Yep, part of each tank was hanging in air. I wouldn't dare do that with a glass tank.
My big concerns are:
1. The possible flex in the long side, 48 inches, of each shelf. I went back and looked at Discus-Hans's racks and he has 5 20G tanks for every span. They are 20 talls so that would be a span of ~5 feet.
2. Will the house floor hold it all! Any home builders or structural engineers out there that want to chime in?