Scubaroo once bubbled...
Assuming non-decompression, non-overhead profiles, and that you and your buddy both have the same additional gas, similar SACs, and you're incorporating using the gas in the 40cf or 80cf stage into your profile, at what point are people doing reg switches?
Breath backgas down to halves minus rockbottom (same as you would if just using a single tank for these types of dives), then switch to the stage? Then, how far do you breath the stage down?
No doubt this sort of stuff is covered in advanced courses like an advanced nitrox, or deco classes, but I don't see any problem in taking along another 40cf of gas if I'm staying under the NDLs for the dives, and it's good practice for future diving where gas switches become necessary...