I'm running Linux Mint 13 with three monitors. The graphics card is an nVidia Quadro NVS 450. Two of the monitors are joined together as TwinView, the other is a "separate X display". Everything displays just fine.
On the TwinView pair the MATE window manager is running, while the "Separate X" session appears to be using the classic Gnome 2 window manager. When setting up the display using nvidia-settings all I did was specify the monitor arrangement, there was no specifying of Window Managers.
I would like to get MATE onto that separate X display, but I've no idea how it came to be in this WM configuration, nor how to change it.