First of all, ensure both Graphics Cards are plugged in to the bluePCI-E x16 3.0 slots on the Motherboard.
If so, check your Motherboard's BIOS. Make sure to select Advanced Mode at the bottom of the screen. You're looking for an option that lets you set your PCI-E x16 slot speeds. They should be set to x16 when you have a single Graphics Card or x8 each when in SLI.