I think you need replace cards and use in config GPU #1, and play with GPU#0 because i think most games will use first GPU in system to render games
-d, --devices gives a comma separated list of CUDA device IDs
to operate on. Device IDs start counting from 0!
Alternatively give string names of your card like
gtx780ti or gt640#2 (matching 2nd gt640 in the PC).
Is there not a other option to set the GPU for rendering games?