I did this with all my win/intel machines under win7. Worked great, although sometimes BOINC gets confused on how much work to request. You have to be sure to use the cc_config.xml option to tell BOINC to use all GPUs. Because the intel GPU and the GPU cards are so different in power, BOINC is supposed to just use the most powerful one be default. In any case, since I have moved on to server intel CPUs, this is no longer an option for me. And there appears no way to do this with Macs that have a discrete GPU.
Code:
<use_all_gpus>0|1</use_all_gpus>
"If 1, use all GPUs (otherwise only the most capable ones are used). Requires a client restart."