ummmm, i sure wouldnt' stack them.
all the programmers crank the pressure up. if you crank the pressure too high, you can actually effectively change the timing, and get cylinder pressures high enough to window your block.
of course if your tuner built your tunes to be stacked, then no worries. but to just simply stack them is pretty much voiding your tuners knowledge of keepign your engine in one piece.
mebbe you are running weak enough tunes it doesn't matter tho.