non-kinematic objects on separate fixed joints not colliding

I setup a simple scene: a base plane+collider, a non-kinematic rigidbody cube fixed-jointed to a kinematic rigidbody sphere (collider turned off), times 2, plus an additional non-kinematic rigidbody cube. When playing in the editor, I slowly move the kinematic spheres around to see what happens: the fixed-jointed cubes follow properly and interact with the plane (pulling away from the sphere) and loose cube (pushing it around) as expected, but they seem not to interact with each other at all; when I move the spheres to overlap, the cubes overlap as well with no collision that I can see.
I would expect the jointed cubes to interact with each other kind of like how they interact with the plane, slightly pulling/rotating away where they pushed against each other. Am I missing something?

 

Maybe the "Enable Collision" checkbox in the joints?

 

That enables collisions between the joined sphere and cube, if I had the sphere's collider enabled. If I turn off "enable preprocessing" I start to get some interaction, but it's no where near what I would expect.