Think of it as a tandem pooping maneuver. Often used when 2 people have to shit at the same time and there is only one toilet. The bottom partner, or foundation, will assume a standard pooping position. The top, or structure, will squat facing the foundation, standing on the foundation's thighs, and hold the foundation's shoulders for balance. The structure shits between the foundations legs. A properly executed skyscraper will result in one uniform plop for both members of the shitting team.
My buddy and I were on a long roadtrip and both had to shit. The gas station bathroom was only a one-holer so we had to skyscraper.