I just tested this, it's very little difference. Even increasing voltage from 815 to 850 still caused the same effect, and still a good 400W below the 1000W max.
I'm trying a new power setup. Different outlets for each PSU. If I'm still having problems, I'm going to switch the two psus around, because they're the same.
With those 2 x Rosewill Hive 1000W, you have 2x 8 pins and 12 x 8 (6+2) pins, right? Then how your GPU power scheme? Also, we need risers power scheme if possible.