The most important factors when considering two-tier benching are ceiling height and room depth. As long as you have enough height for sitting comfortably on the top bench and enough floor space to move around, two-tier benching allows for environment variations within one enclosure and is a wonderful enhancement to any steam shower.
Ideally, you would construct 24" deep benches at 18"-20" and 36" off the floor. Be sure to use a smooth surface material such as marble or granite and slope the benches a few degrees to keep water from pooling on the benches and for overall bathing comfort.
For more detailed information, read about comfortable seating