Have you tried starting them both at lower clocks, then slowly tweaking up? Sometimes the start up surge causes momentary voltage sags as the PSU suddenly has to deal with another 150W of demand so kills the stability.
I'd love to know more about this. For me I've found I do better if I start from cold and dive straight into a high clock rate. It may be a good idea to make my idle voltage the same as my performance voltage to ease the transition. I've found that changing clocks while mining can cause the drivers to freeze if the clocks are high enough.
I once left my good card running at 1025 MHz at stock volts for 12 hours and it was fine. Then I change the clock rate to 1020 while mining and it crashed instantly. I reset and started it at 1020 MHz and it was fine. Indeed, I can often get 1030 MHz like this and it runs fine once it's there. I can sometimes get 1035 MHz like this but even if I do the card will crash after a few hours. I get more and more kernel warnings the higher I take the clock but even at 1030 MHz the extra 5 MHz is a greater boost to profit than the hardware errors are a drain.
Perhaps this has something to do with temperature. When the cards are at idle they are at much lower temperatures and can probably take the jump more easily. I've found that if I turn my fans up I can reach higher clock rates.
I wrote a script to change my cards clock rates on the fly if the temperature left a certain range but all too often it would crash the cards so I stopped using it (I wish I could control my fans in software).
Anyway, I'm certainly going to increase my idle voltage from 0.95V to 0.9875V so that at least the voltage isn't changed when I start/stop mining. This could be a good stability tip for people who do pool mining and occasionally have no work to do due to server issues.