Just spitballin here but I would think spring pressure shouldn't be a factor as long as both guns are setup at the same velocity. Sweetspot both regs so each gun gets what it's going to get then shoot the same paint, bore, etc.
How tough is it to accurately fill a 13ci tank? I was borrowing a small tank once and the toolbags at the field over filled it *quick*. Not that I can think of a better way to do it...just asking.