In v5, running two instances does not give more S/s (at least in my experience). This could be a way for improvement. in v3 running second instance was bringing +25-30%.
If I look at the new competitor Optiminer, he's results with two instances show what you could obtain with SAv5 + 25% with second instance.
Good luck and much thanks for good work.
This is actually an indication that GPU is now fully utilized. Additional optimizations are going to take a bit more work now.