For a trike you could build a ladder into the frame (more weight, but who cares?!!) Stability would depend on how big the base triangle was, but the bottom rider would probably have to mount first in any case. Trikes get most of their stability by having the weight down low, so a two-story trike of normal track and wheelbase would be very susceptible to tipping over in turns, and maybe even in a straight line if the road had enough of a crown.