I think we should make the sum the two forces 50+50=100 N and say it can't handle? Can you explain me using the 3rd Newton Law?
This question comes up again and again, in different guises.
If you pull on a rope with a force of 50N, then the tension in the rope is 50N, not 100 N. It's no different whether the other end of the rope has a weight of 50N on it, or whether it's attached to an anchor point in the wall, or to another boy.
Newton 3 talks about the fact that A exerts a force F on B, then A must feel that force too. A cannot feel more or less than the force he is exerting - that would be a short path to violation of conservation of momentum (if A and B are both floating in space, then $F\Delta t$ for A must be equal and opposite to $F\Delta t$ for B so that their total momentum is unchanged; and since they feel the force for the same amount of time, the direction of the forces felt must be equal and opposite).
Nothing in the above doubles the force.