I think your bug report is the one that made my mind go

The CUDA constant memory (the c_N loop trip count, etc...) of most CUDA kernels is only initialized properly for the first GPU (use of a single static variable to mark initialization instead of a thread-specific static variable). Which explains the majority of the crashes people are seeing with multi-GPU. Thank you. The Fermi owners use a kernel that doesn't yet make use of such constants, and hence the multi-GPU support is working fine for them.

So this is also on the FIXME list for tonight.

However I think that in your case where you run two cudaminer instances this cannot be the root cause. So we will have to keep looking.

this is a long shot: Would it help to downgrade the video driver to the exact version hat shipped with the CUDA 5.5 toolkit download from nVidia? Of course if your video cards are newer than this driver release, this is a no-go, as the driver would not recognize them ;-/