Like I said, the limiting factor isn't the lifter's ability to hold extra weight up, it's in the execution of the jump. I disagree with your view on the main lifter, though - the back lifter is doing the bulk of the work. See Beast when he is used as a solo lifter - he is always behind the jumper.
Solo lift is different to a two person lift.
In a modern two person lift, the rear lifter is primarily providing stability at max height under the backside of the jumper, or even on the upper thighs. They help lift, but the main work is done by the front person who will be binding on the lower to mid thighs, either on the inside thigh only or on both thighs. Generally, it is more effective if the taller person is behind the jumper rather then in front.
In a solo lift the back lifter will be binding higher around the jumper, such as around the waist. This way they can provide both more effective lift and stability. Thus, the height of the jumper will be lower than with a two person lift. A solo lift will also struggle to keep the jumper at max height stably for any amount of time.
That's my experience from both jumping and lifting in the lineout. I think a few people here still play rugby and jump, and so could clarify further. Personally, I hated lifting in front as the bind was harder on the thighs, particularly if damp, greasy and sweaty.
