2nd video card, blank screen?

[Jeremy_Ray]

Here's what's detected -

  *-display               
       description: VGA compatible controller
       product: Barts XT [Radeon HD 6870]
       vendor: Hynix Semiconductor (Hyundai Electronics)
       physical id: 0
       bus info: pci@0000:01:00.0
       version: 00
       width: 64 bits
       clock: 33MHz
       capabilities: pm pciexpress msi vga_controller bus_master cap_list rom
       configuration: driver=radeon latency=0
       resources: irq:44 memory:b0000000-bfffffff memory:fddc0000-fdddffff ioport:be00(size=256) memory:fdd00000-fdd1ffff
  *-display
       description: VGA compatible controller
       product: GT218 [GeForce 8400 GS Rev. 3]
       vendor: NVIDIA Corporation
       physical id: 0
       bus info: pci@0000:04:00.0
       version: a2
       width: 64 bits
       clock: 33MHz
       capabilities: pm msi pciexpress vga_controller bus_master cap_list rom
       configuration: driver=nvidia latency=0
       resources: irq:19 memory:fb000000-fbffffff memory:c0000000-cfffffff memory:de000000-dfffffff ioport:cf00(size=128) memory:d0000000-d007ffff

The Radeon is the first card and is running two monitors more or less o.k. The Geforce is the new one, with a third monitor that is displaying a blank screen.

I followed the instructions here to configure the Geforce - Link

Systems Settings shows nothing for the third monitor.

Help?