In the xorg configuration, you can specify the display card by PCI-e ID, something like this:
VendorName "NVIDIA Corporation"
BoardName "GeForce GTX 275"
Option "Coolbits" "1"
and then delete any “Device” section for the compute card. There is a utility called nvidia-smi which can then be used to set the compute mode of each card, you can set the display card to “compute prohibited”, and the driver will not permit a CUDA context to be established on the device.
It is irelevant, both because CUDA doesn’t have anything to do with SLI, and because with two dissimilar GPUs you can’t use SLI anyway (not that the GT210 supports it).