there is a script you can build and run to make it quick:
1) Open notepad
2) Copy the following in:
Option Explicit
Dim WshShell
Set WshShell = CreateObject("WScript.Shell")
' Open the SLI Mutil-GPU applet.
WshShell.Run("rundll32.exe nvcpl.dll,LaunchMultiGpuApplet")
' On slower machines this Sleep value may need to be increased.
WScript.Sleep(500)
' Send the keystrokes to change the mode.
WshShell.SendKeys("%E")
WshShell.SendKeys("{ENTER}")
WScript.Sleep(500)
WshShell.SendKeys("{ENTER}")
WScript.Quit(0)
3) Save the file as "SwitchSLI.vbs"
Note: Be sure to change the drop down box from *.txt to All Files.
4) Run.
you run it to turn SLI on, and run it to turn SLI off (for dual monitors).
otherwise, you have to turn it on and off in the nvidia control panel and re-boot.