Depends on the specific mobo; I have listed all the mobos I have tested on the OP.  I am sure it works with many more, but right now I recommend only using the ASRock H110 PRO BTC+ (13x gpu) mobo, and selling your other mobos to gamers.  I still have to do this myself.  I'm in the process of switching all my rigs to 13x now myself.

Does it reboot twice in a row, or only once?

Also if you are connecting a monitor directly to the motherboard (don't do this); only connect a monitor to the primary GPU (the one in the 16x slot closest to the CPU)