I'm looking to do about the same thing. I have a drilled 180 that I would like to stack on a 125. I don't know if I will use the 180 as an overflow just yet; I like the idea of having the 125 for use as well.
I'm questioning whether or not 4 4x4 or 6x6 will support the load. The problem is whether or not center braces will be needed. At the very least, a rear brace can be installed and not prevent moving of the 125 beneath, but how do you go about adding extra support to the front and prevent any bowing?