You can definitely do that, but it would be nice to have a single cage that is that big. The only problem I could see with it though is that you would still need a brace in the middle of the 4'x4' panel which would be huge and if it was the front that would be an even bigger door to open which would be kinda crazy.
I think the 2 cage idea is a much better way to go about it and you can use little metal brackets to screw them together no problem.