How do I know both cards are working?

I have a pair of 8800GTX's running on my phenom in SLI.

I got CUDA working, but it only shows a single card. How do I know both are working?

You need to disable SLI to get both cards to crunch.

The only sure way of knowing both are crunching is to download Rivatuner and go to hardware monitoring to see the temperatures of the cores. If they are close to being the same, they're both working. If one is significantly cooler, then only one is working.

You should see one CUDA workunit crunching per GPU within the BOINC client.

For example, a quad core CPU with two GPUs will have a total of 6 tasks running. The two cuda units will be designated by a "(cuda)" at the end.

You will need a dummy plug on the second card and the desktop attached to the second ghost card in order for BOINC to see it and use it.