Rendering using both GPU and CPU is not possible For some reason I'm unable to render on CPU+GPU. I've checked on 2 different computers and it's not available. One setup is Threadripper + GTX 1070, and the other one Intel i9 + GTX 1080.

Is it a matter of supported processors or I have to activate it somehow?
I'm using Blender 2.79b
In fact it IS possible. You need to download a branch like this:

http://graphicall.org/1240

...and many others on this site. Open Settings and in the System pane You will find settings as on the image below. Hope it helps, buddy.

Sincerely, Jan

REM: Sometimes using GPU+CPU could be SLOWER then using GPU only!