I think I've seen some amazing Blender hacker put Cycles to the test on a machine with both NVIDIA and Intel GPUs. Love it that their API seems that portable and able to parrallelize on heterogeneous hardware. Amazing software work.
Getting blender to run on my NVIDIA GPU and AMD CPU simultaneously is as easy as checking two boxes in the settings. It's not usually worth it since the GPU absolutely smokes the CPU. it's a testament to how well blender is made that it works at all, let alone that trivially.