You don't say how far apart you are going to stack them, but my guess is
that you wouldn't want any of the elements on adjacent antennas to be
less than about 6 feet apart. Also, although I know lots of folks get
away with stacking beams on tall masts, my calculations show that even a
thick-walled 2 inch high strength steel mast is marginal for stacking
small beams more than about 10 feet above the tower. I guess it depends on
how often you want to replace the mast after a storm. In any case, get
the strongest mast you can find.