Hi there,

I had the same issue when i bought my super rig with 2x 280GTX's.
There is a crude solution, and that is to set your primary monitor in display options to your second physical monitor, load the slave sessions, change the primary monitor back to the original and launch your main session and that'll set you right.

I have no idea where this restriction is coming from considering how easy it is to get around. I have 3x monitors, two connected to 1 card, 1 on the other, running 5 sessions. As long as you don't 'cross the beams' so to speak with moving sessions from one cards display area to another (2 monitors on the same card is ok.. assuming there's two dvi ports), you'll be okay.

Enjoy!