My old cage was 2 all living things cages zip tied together (one on top of the other with the top door removed) It worked fairy well, was a bit bulky and awkward and hard to remove from the pan but I made it work. I don't know much about the rat manor, but could you put two on top? I bet removing the side panel would work too, it would just be more technical. My boyfriend calls our old cage Frankencage, haha.
What about snipping a couple holes in the sides for the different levels and zip thing them side by side? I am trying to remember the exact construction but I'm not sure if removing a side Panel could work,..
Well the side panels can be folded up, and I thought that using Zip-ties on one side, and binder clips on the other would work so that I could fold it back when I am cleaning and get better access to the cage.