Welcome to the ROG forum.
There really is no secret, try with one gpu only in the pcieX16_1 slot, connect the proper pcie power cables and monitor cable to the gpu. Fire it up and it should give you display to enter the bios.
If the gpu isn't recognized or giving you display, in the bios, double check the Primary Display setting is set to Auto or PEG. Or try resetting the bios to defaults with F5.
If you're still having issues, try updating or reflashing the bios.
Does any of this help?
What code does the motherboard Q-Code stop on?