As post 12 has discovered, most modern tandems come in sizes, usually only 3, small, medium, and large. They also come in combinations of those sizes for both captain and stoker. Ours is a medium-small. We both have standover, but neither of us can reach the ground sitting on the saddle, which is normal. We both have standover because the straight top tube slants down toward the rear of the bike because it's a medium-small. However most boob tubes are level and thus a shorter stoker with shorter cranks than captain's will find their toes further off the ground than captain's. That's normal.

Actually in the Proper Method, captain mounts first. Stoker mounting first makes no sense unless the captain can mount by swinging a leg over the bars, even then it's questionable. Captain steadies the bike while stoker mounts cowboy style from the pedal, not the ground. Stoker of course clips in first while captain holds the brakes on to make that easier. Stoker offers the captain a pedal, captain clips into it, releases the brakes, and they're off.