As OpenCL is a bit trash right now on blender cycles, we could use some solutions for faster rendering. I render with bigger tiles as AMD can only render fast on bigger tiles but I also enable the CPU cores so I have big tiles on the CPU. But the problem is that the CPU tiles can take a long time to finish while the GPU is finished and does nothing.
It should be possible as both use OpenCL and has to do the same calculations? And that overtaken CPU thread just take over the next tile. Rinse and repeat til the image is finished.