Sorry Max but you got that part wrong. It is very easy, and my preferred setup, to run one ATI/AMD & one nVidia on the same machine. I never have to worry about matching GPU family types/series, never have to add any special flags to cc_config, etc...
Also I never have to worry about any machine being completely idle if one type of GPU project runs out of work, has a new app that fails, the server crashes or just goes dark for an unspecified period of time.