Have you tried lowering the frequency? 800 might be too much for dual mining. Also, make sure your power supply is supplying enough power. Back before cpuminer could disable the BTC cores my power supply was not powerful enough and the same thing happened, no found shares.
I played with frequency all the way down to 600, still the same results sha256 mines fine all the way up to 900, scrypt no shares being made, psu wise it is hooked up to a 850 thermaltake PC PSU, lots of juice there
What happens if you launch cpuminer only? I was getting those same results... when I get home I will see if I can recreate it.