To resolve this problem, set the total screen size on the command line for XMing.
For instance, I use two monitors of screen resolution 1280×1024 where 1280 is the screen width and height being 1024. So, for the dual monitor to get working, I should set the screen width as 2560 (1280 x 2) x 1024 on the command line as follows:
xming.exe :0 -ac -clipboard -multiwindow -screen 0 2560×1024
-ac : forces the use of Access Control List
-multiwindow: starts xming in Multi window mode
-clipboard: allows access to the system cipboard
-screen 0 2560×1024 : sets the screen 0 size to 2560×1024
If you are using the desktop or startmenu shortcut then edit it by right-click and properties and add the “-screen 0 2560×1024″ to the end of the command line. Stop and restart Xming with the new command line options. This should do.