First you need to verify that both of your PSU's are working.  If you have 24 pin short plug, test the PSU individually.  If you don't jump pin 16 and 17.  If they both power on then move to the next step below.

If both power up, then next step is to remove all of the everything from the motherboard (ram, pci cards, drives that are connected.)  Clear the cmos and then plug in just one power supply, 1 stick of ram, keyboard and plug into the onboard video (dvi-d port).  If the system powers up, then you know the motherboard is good. 

Then start checking your gpu's one by one by plugging them into the PCI-e 16x slot on the motherboard.   If they all check, then your gpu's are good too.

At this point it comes down to either riser cards and or the pci-e 1x usb cards.

So plan on spending time testing each one of them.

Jims2321