Clicked on this tiny "More..." link on AsRock's main page for this H110 motherboard...brought me here.
http://www.asrock.com/Microsite/H110ProBTC%2B/Look at the screenshot of the Device Manager in Windows. Ummm....is that 13 GPUs?
Also, look at Tip 4. Do you think it is safe to have PSU-2 supply power to both the risers and GPU of the second set of cards? I thought it was safer to have the risers powered by the same PSU powering the MB.