Not having done a turkey... yet, presentation aside, could you not take a frozen turkey and apply a chain-saw to cleave the sucker in half? I'm guessing the halves would fit on a single grill, with a much less height requirement?
Again, presentation aside, you could cut the birds into various parts, legs, breasts, wings, thighs, and place on different grills, at different times to try to get it all cooked at the same time?