The general rule is that the magnitude of a force of type F on the compound is the sum of the magnitudes of the forces of type F from that agent on the parts.  This sum rule is needed to handle cases where the same agent may have an interaction of the same type with several of the bodies.  For example, if two blocks are pushed side by side along a plane, the net normal force from the plane on the compound is the sum of the normal forces from the plane on each of the blocks; similarly for kinetic friction on the compound.

As another exmaple, in the case of weight, the magnitude of the weight force on the compound is just the sum of the magnitudes of the weight forces on the parts.

It is important that the forces acting on the part not only have the same type but they have the same agent.  For instance, if string A is attached to block 1 and string B is attached to block 2, then it doesn't make sense to define a tension for on the compound body due to both string A and string B.  A force an only have one agent.  However, if a man pushes on both blocks 1 and 2, then we could define an applied force on the compound body due to the man.