SSS render problem - how to fix without joining meshes?

Subsurface scattering seems to behave quite differently with intersecting objects if they are in a seperate mesh. Take ths simple example of a plane intersecting a cube as an example:


Notice the add “Ambient Occlusion” type effect where the two objects meet. I am not using AO, and this looks like a strange artifact of the way that the SSS is being calculated. Now if I join those two objects into one single mesh, then the SSS calculation seems to be a little more sensible (but still a little weird perhaps?):


I tried building a 3D object that represntes the union of two intersecting objects, and the render results looks to be somewhere inbetween the two above:


Is there any way to get the same result as joining meshes, but without actually joining the meshes? I would like to preserve the modifiers set for each object, without having to apply them all to avoid the modifiers affecting the new joined parts of the mesh.

Also, as a bonus question, is the strange looking dark bits at edges and points, actually something we would see in the real world for an object that exhibits sub surface scattering? If it’s not a realistic phenomena, how can we stop it from happening?

Thanks!

Matt