Update: I was wrong. The problem was I set something wrong in RBE. Powerstate voltage should not be changed, otherwise GPU BIOS would treat it as invalid value - that's why one card dropped to 400MHz (and in fact it was both cards dropped to 400MHz. The "working" one was only an display error).

After everything was fixed, my cards could stay stable at 970MHz/1.25V, the whole system is draining 497W +-3W from wall. (1200W PSU, 80+ Silver)