I just did this with 3 machines and this is what I did/found.
1. If you can get it to work under Vista you are a smarter than I am.
2. Install ATI into slot 0 and get it totally working ... including BOINC.
3. Stick in Cuda card with NO dummy plug/monitor and don't extend desktop.
4. Load the Nvidia driver. I ONLY installed the display driver not complete pkg.
5. I put use_all_gpus>1<use_all_gpus in the cc_config.
6. I'm controlling both cards with MSI Afterburner
Works under W7 with 5970/GTX 570, and 5870/GTX 570
It works with CAT 10.10, 10.12, 11.3, and 11.5. Nvidia 266.58