I've currently got this running using the method described for setting up a dedicated nVid PhysX card, whilst ATi does the rendering
Basically, it involves 257.21 drivers, plus the "GenL PhysX mode 1.04ff" (search
) and it lets the nVidia work without dummy plugs or fake desktops or any of those shenanigans 
just need to figure out the correct use of the variables -gpu 1 / 0 or force_gpu etc
mine is -local -forcegpu nvidia_g80 for nVid
and -local -advmethods -gpu 0 -forcegpu ati_r700 for ATi
Basically, it involves 257.21 drivers, plus the "GenL PhysX mode 1.04ff" (search
) and it lets the nVidia work without dummy plugs or fake desktops or any of those shenanigans 
just need to figure out the correct use of the variables -gpu 1 / 0 or force_gpu etc
mine is -local -forcegpu nvidia_g80 for nVid
and -local -advmethods -gpu 0 -forcegpu ati_r700 for ATi









