Well if you think about it as in them sharing the render… but you could indeed do 2 separate renders as long as everything is in the scene it should be the same CPU or GPU.
Think of it as in render layers send 1 off to GPU and one off to CPU which should not cause any performance hits (not much anyways if your CPU is being maxed out everything will be a little slower)
Here is a scene that could use that
If we take the cubes that are sss and mask them out we can have the CPU render the floor and GPU render the cubes at a much higher sample count
We can do that now if we wanted but it would be nice if there was a nice system to set that up for us aka a drop down box in the render layers to render multiple renders at the same time and pick which device you want it to render on and tile size