How to access second monitor from another machine remotely which has a single monitor?
It's more like a workaround: In the remote connection window on your demo pc, use Alt+Tab to bring the window to the front, then press Alt+Space, choose "Move" on the popup menu, then use the arrow keys of your keyboard to move the window until you see it.
Dual Monitor without remote Desktop:
• Use ( Windows Key + Shift + Left/Right Arrow Keys) in order to move from one window to another window in Dual monitor.
Dual Monitor with Remote Desktop:
If you are using Remote Desktop on dual monitor, do following steps in order to move using keyboard
• First, use (Ctrl + Alt + Pause/Break) it will select and resize your Remote Desktop.
• Use (Windows Key + Shift +Left/Right Arrow Keys) in order to move from one window to another window).
• Use Ctrl + Alt + Pause/Break) it will select and full screen your remote desktop.
You can either move your application like the above answer mentions
(alt+tab) - to select the application
(alt+space) - to open options
(m) - to start moving
this will not work if the application is already fullscreen. In that case,
use (alt+space) and (R) to restore the application
and then use (alt+space) and (m) combination to start moving
Another way (not sure if this will work on < windows 10) is to use
win+shift+ctrl+(left/right) after selecting the application
If using 3rd party software is an option, I think TeamViewer 8 supports having multiple displays on the remote end point.
Edit: For the options without having any 3rd party SW, after doing some more googling, there seems to be support already built-in in mstsc.exe using /span or /multimon switches. Depending on the version of Win7 you're running, /multimode might or might not work as "multimon feature only works if the remote controlled computer is running Windows 7 Ultimate/Enterprise or Windows Server 2008 R2 Standard/Datacenter/Enterprise" (quoted from the previous link). Furthermore, since you're connecting from WinXP all this wouldn't work out of the box, but you need to install RDP 7.0 client on the WinXP (which requires WinXP SP3).