NX 3.5.0 supports multi-monitor solutions.

On Windows

NX Client for Windows allows to configure the desktop session to allow it to spread over multiple monitors when the display is set
to 'Available Area', 'Fullscreen' or 'Custom' size. In the case of single-application mode, the session is automatically spawned over all the available monitors.

Sessions running with Unix/Custom desktop settings will be displayed correctly only if both monitors are set to the same color depth.

To indicate which monitor of your set-up you wish to have the NX Client window open, you should first move the NX Client GUI to the monitor you want to have the window opened in, then start the session normally. The session fullscreen window will be created on the monitor where the GUI was placed.

Related feature request is:

http://www.nomachine.com/FR11F02124
 

On Linux

In NX Client for Linux, the session is automatically spawned over multiple  monitors. This however could have some drawbacks since, from the nxagent  point of view, multiple monitors are perceived as a unique monitor. This means  that if, for example, an application is iconized on monitor A, it is iconized on all  the available monitors.

Improvements to the multi-head support for Linux displays are available with NoMachine 4 and later versions.