The force is proportional to the mass and the acceleration or in this case the deceleration of the moving objects.

The deceleration (a), of one plane is the change in its velocity (dv),divided by the collision time, (dt) makes this equation:
a=dv/dt.

The same is true for the second plane. The force on each plane is its mass (m), times the deceleration (a):
F= m*a.

And the sum of the two forces is the total force.