I've just built a fresh Ubuntu 12.04 AMD64 machine. Box is a HP XW8600 with dual nVidia Quadro 1500's gphx cards (not SLI but mobo has 2 x PCIx16 slots). There are 4 x DVI outputs for multiple monitors, 3 are polulated with identical HP 24" monitors on DVI cables. After some troubles with X server, I installed the "nVidia Current" driver and tweaked the nvidia-settings as root. X server now recognizes the 3 monitors and the mouse will transition happily across all 3 screens. I'll call them screens 1, 2 & 3.
Unity happily displays on Screen 1, but don't show on 2 & 3. The light grey colour of X server is on screens 2 & 3. The mouse appears as the typical "X" when moving over screens 2 & 3, yet is the regular "arrowhead" when on screen 1 in Unity.
Q - how can I make Unity show on screen 2 & 3. I'd prefer to have separate screens (like you'd see with Twin-view) but would also like to test Xinerama for 1 monster-long screen.
The "Display" in Settings is only showing 1 screen called "Laptop" which I can't alter, but the nVidia-Settings are showing all 3 x HP monitors correctly.
Once I've sorted this, I'll most likely transition from Unity to Cinnamon desktop.
thanks in advance