Look, it's simple. The upper rider simply steps on the stomach of the lower rider and the lower rider then raises his knees so the upper rider can then step on his knees to get up to his or her seat. You think I didn't think of everything?
As for the high center of balance...does it have to be that high? What about making the upper seat just as high as it needs to be in order to give the lower rider room for his reclined body and his pedaling motion, and no higher. Would that really be too high? I can't really picture it, so I'm just asking. Seems to me that you wouldn't need that much room.