Problem using separate pedals to the wheel base?

I use a Fanatec wheel base but I use a different company's pedals that I just connect straight to the PC with a USB (VRS pedals). GRID detects both but I can't seem to assign the VRS pedals to my throttle and brake? The game detects the pedals and when I press it an axis appears so it doesn't seem to be a problem detecting it but I can't seem to use the pedals with my wheel base to work together at the same time.

Could I get some help on getting these working together?