Well, you're lucky running two dual channel kits on the RIVE... there are a lot of people having problems with that...
The safest way to choose a memory kit for a mobo is to choose it from the QVL. That's not meaning that others would not work, they just can not be suggested with 100% confidence... - that's it
So, if you want to try, go with the quad channel kit for the Rampage III Extreme too... who knows?... you may be as lucky as with the RIVE...
All what I tried to say is that you're wasting money since you won't use one stick; and if i would be you, I would go with a triple channel one from the QVL.
🙂