Which position do you think makes for the greatest difficulty? My guess is the guy on the top since the guy on the bottom give a lift to the guy on the top. Perhaps they switch positions after a certain number of reps.
We had to do a 4 man variant in boot camp, where we were arranged in a square. Its extremely difficult. Its a team building exercise as everyone has to move together to successfully complete the exercise.
If I remember correctly, it was harder having the guys legs resting on your back as you had to lift him as well as yourself.