Sadly they are not interchangeable, the GQ series seems like the odd one out as you can't even buy the cables from the evga site.
As for OP, what you can do is swap the psu end of an 8+8 cable with the psu end of one of your evga single 8 pin cables. You have to make sure the wires are aligned exactly as your other evga pcie cables. There's a youtube video of how to remove pins off a psu cable using staples if you want to give it a go.