I like the idea, as long as your under 13'6 you will be fine.
I have lifted the $hit out of my bus. 23 inches roof raise, and 12 inches of lift on the coach work.
I will be hanging all my real heavy stuff down low, so center of gravity will not be a issue. Also I will just pull over in windy weather, and drop the stabilizers.
I considered mounting a glass roofed minivan up there because I love laying in bed looking at the stars. But the van would be too high with my other lift mods, so I had to let that go.
If I was you, I would start by lowering the entire bus on it's chassis. Depending on the style of bus, you would need to lift the engine so it won't drag the ground, then slam the chassis withing a few inches of the frame touching the rear axle. This will buy you anywhere from 6 inches to a foot more useable space on top.
Now using two curved roofs will just waste space. So I would drill every rivet out of the chair rail, and lift the entire body of the bus off the floor. Then using the same method as a roof raise with formed hat channel, extend the body height 4 feet.
Now the simple part. Measure down 4 feet from the peak of the roof on each side. Again using formed metal for the new second level floor joist, attach them to each side of the support ribs at the 4 foot mark. This is your new second level floor, and the new ceiling for the first level.
Now you have a nice flat, easy to finish ceiling on the first floor, and a 4 foot attic for the kids with a flat floor.
Are you really up for this much work? Most take years to complete a simple conversion. Metal work takes forever.
"Don't argue with stupid people. They will just drag you down to their level, and beat you up with experience."
Patently waiting for the apocalypses to level the playing field in this physiological game of life commonly known as Civilization