We'd have to know more about the scene. Are you rendering with both GPU's? Also RPR tries to load balance between the two but that might not be happening great in this case.
I've tried different scenarios now.
Even though windows tells me about a gpu-usage about 1-2% for the vega64 it renders every scene i tested about 2-3 times faster than the gtx980 with a usage of 70-98%.
Sadly it does not make a big difference, if i render on both gpus +cpu. in the last scene i tried a testrender with 100 iterations: it just brought down the rendertime from 1:19min to 1:11min.
Thanks for your great work so far - i can't wait for further versions of rpr!