It`s really strange. U have good power supply and with such usage - u are using just about a half from it nominal power. But I don`t have any ideas, what else can doesnt work. Just to try different PCI ports at motherboard.
Both GPU test fine by themselves. The system will mine 24/7 no problem with just one.
I have a power supply on the way to test with over the weekend. If that doesn't solve the issue then not much else to try but replacing the motherboard.