The typical metal framed store bought double tank racks have racks for the same footprint tanks...
But you could put a board on the bottom and put smaller tanks if you wanted to... such as a 55 gal on top and two 10s on bottom...
I was thinking of stands for the 180 gal (6' x 2' x 2') and 120 gal (4' x 2' x 2' or 60" x 18" x 27") that you mentioned. It is exceptionally unlikely that you will find a store bought stand that will suit these two tanks that is not custom made.
The store bought metal framed stands that will be able to hold either of these tanks on top, will have a lower rack designed to hold a tank with the same footprint. But yes you could put plywood or somthing similar on it and put smaller tanks... or even tanks with the same front to back measurement but shorter.