NsCDE has a basic support for the multiscreen setup which is basic as it is FVWM multiscreen support, with couple of menus added and functions dealing with move and resize operations which are aware of the multiple logical screens. Single logical screen is referred as "SLS" in FVWM and NsCDE documentation. Screens are implemented (and this cannot be changed in FVWM) on the sub-page level. In other words, as workspaces (desks) contains pages, pages are split to two or more monitors inside one single page. This can be a bit confusing in the combination with edge scrolling and window positioning and it takes some practice to become comfortable with such third, non-trivial space on the screen which is already divided logically in two levels.
Monitor handling by the Xrandr X extension is for now out of scope for NsCDE and is dealed with xrandr(1) command and other such tools. Nevertheless, when other monitor is added to the system in SLS mode, FVWM/NsCDE must be restarted (restart session simply) to recompute spaces, screen sizes and so on. After restart, two new menus are available: One on Root menu on which there are entries to move all windows on current screen, or to pick a window for moving to some of the (xrandr identified names) logical screens connected to the system. Second menu will appears on the "Window Options" menu called from the first titlebar button of the window or from the root version of the "Window Options" menu. This menu allows moving current window to other logical screen.
When logical monitor is disconnected from the X setup, FVWM NsCDE must be restarted again to get things right again.
Front Panel will appear on the primary screen, but can be moved to other screens by Ctrl+Escape pressed while pointer is on the desired screen. This does not work always well when logical screens are of different resolution and it is specially visible when making third mouse click on the Workspace Manager buttons which can be popped down below the screen instead of up to be visible. On the monitors of the same resolution in SLS configuration, no such problem exists.
All other functions and window positioning managed directly by the NsCDE will handle windows and transient windows correctly, so no windows centered between two screens are expected, but some barely visible flickering and quick moving can be observed by some parts of the NsCDE in some cases. For example, PGM - page manager left down from the Workspace Manager on the Front Panel when clicked will popup "Go to Page ..." menu in a more free floating form, and not directly above PGM dynamic icon and such things ...