Pump -> GPU2 -> GPU1 -> Dual Rad -> CPU -> Single Rad -> Pump is an option.
This path would help keep the two GPU's nice and cool by getting "fresh" cool air coming into the case. Then, that heat is dumped by a "dual rad" to cool the CPU.
Question: Are you using a reservoir in this cooling loop? Or, is the reservoir built into the "dual rad" since it's located in the highest place like a Koolance setup?
Your layout will work, however, any air in the system would find it's way to the "dual rad." Having a fill port or even better, a reservoir box, located off of the rear "dual rad" would help greatly to remove trapped air. Just a thought.
Stev