From
Corsair's website:
Radiator Compatibility:
240mm: Front only
120mm: Front, or Rear
So in theory yes. If your CPU radiator is a 120mm, I'd suggest putting that on the rear,, as I'm not sure how well you'd be able to route the GPU's cooling tubes there.
As for two closed loops in one system generally, yes that is definitely possible. I've got an EVGA 1080 hybrid and a closed loop CPU cooler in my PC.