Whatever amount of force each person exerts on the box, THAT's the amount of force the box exerts on him. There's no reason the forces of each person have to be equal.
Let's say . . .
-- Mr. Smith pushes on the box with 499.99 newton's of force.
-- Mr. Jones is actually a fly; he pushes on the box with 0.01 newton of force.
The total of their forces is 500 Newtons on the box.
The box exerts 499.99 newtons of force back on Mr. Smith, and 0.01 newton of force back on Mr. Jones, the fly.
