You may have to dial the core settings down on the second one. My Sapphire Vapor-X 5870s went crook after a while (came from a bad batch) and I had to underclock them to not get them to crash.
I thought of that, but nothing is overclocked and they seem to be running fine. But the mining software all crashes on startup. It's probably driver related, but I can't gleam any useful info.