Is your CPU OCed?
Do you have PCI-e in the BIOS set to 100MHz?
GPU Temps?
MSI AB should OC multiple series GPUs. Just make sure the synchronize GPUs box is NOT checked, and you set them individually. Change to the GPU you want to adjust through the settings.
On that chipset, you prolly have what... a 4x slot for your second card? And, your first slot probably drops down to 8x. Doing this kind of work, you are probably saturating these slots.
Additional note:
Install each individual driver for each card.
CPU is not OCed, temps are 72, PCIe is at 100Mhz.
Actually mining is not memory intensive. That's why people here can use 1x extenders without a problem.
Just my two cents, but seems mixing isn't good.
I'm downloading LinuxCoin right now. See if it runs in there.