Newton's Third works fine for me.
Check that your Drag and Friction values aren't high. My guess is that ent1 has no D or f, while ent2 has alot of D and f.
Check that ent2 is also registered with the PE. If you have anything but PRO, you are only allowed one registered entity.

Either your physical parameters aren't set correctly or you don't have PRO.
Other than that, I dan't see why it shouldn't work