How to use both CUDA GPUs?
The -gpu=X command line is telling it which of your CUDA devices it should use, not how many it should use. Like most things written by programmers, the index starts at zero.  If you have two video cards, run it once at the command line with -gpu=0 and then a second time again with -gpu=1.  Once you work out your ideal fine tuning on aggression, gpugrid and gputhreads (if you do at all), it's easiest if you make a batch file (unless you're using GUIMiner, in which case create a new CUDA miner for the second card, same extra flags on both other than gpu=0 or 1).

You can execute it twice without specifying the GPU and it will certainly run, but from the console output it appears it assigns both of them to the first device.