Hi:
In my experience dual mining consume more power and stress more the GPUs, you could need to increase a little your voltage if you are undervolting around 875-925 mV, check the log file for the GPU that hangs, then add 10mV at first to the settings of that GPU, keep trying until you get no hangs for 24 hours, you could decrease you clocks too, I am guessing here that you are using a modded GPU.
If there is no hanging in the log file then it must be the power supply.
Good luck,
Carlos
Thank you, i tried already different voltage settings but always the same, the system crashes within seconds whenever the miner begin to mine 2 currencies .
What i have
not tried yet, an older Claymore version, another driver and a test run with only one card.