The difference being, pushing the limits of an OC'ed card locks the system doesn't reboot it like this issue is causing. With no OC'ing It has been working for about 18hrs so far without rebooting itself.
That's why I suspect it's your PSU. Because an excess OC'd like you said, should just result in corrupted display or a system hang. Reboot sounds very much like a PSU issue. So the only way to find out is to reduce system load, by running one card, and clocking as high on the other as possible. If you're able to reach much higher clocks on either of the card alone for much longer than when both are running, this means you know at least it isn't because the GPU core isn't able to handle it.
Since temperatures for both cards are relatively cool even running together, that rules out overheating of the card.
So you're left with the PSU being too weak to do this for extended period.