How to make two mice work with xwindows (x.org)

It’s a real pain to surf the Web when the batteries die in your wireless mouse or trackball.  I use some old rechargeables that are no longer fit for digital camera service, so this happens to me fairly often.   My batteries just died again, so I had to figure out how to configure X.org to use a backup mouse.  It’s actually not hard to have an old 3-button mouse plugged into the PS/2 port and tell X.org to use it as a backup.  First, define two input devices:

Section "InputDevice"
	Identifier  "Mouse0"
	Driver      "mouse"
	Option	    "Protocol" "auto"
	Option	    "Device" "/dev/input/mouse0"
	Option      "Buttons" "7"
	Option	    "ZAxisMapping" "4 5"
EndSection
Section "InputDevice"
	Identifier  "BackupMouse"
	Driver      "mouse"
	Option	    "Protocol" "auto"
	Option	    "Device" "/dev/input/mouse1"
	Option      "Buttons" "5"
	Option	    "ZAxisMapping" "4 5"
EndSection

The first mouse is my wireless Logitech trackball (which is currently a fancy paperweight while the batteries charge).  The second mouse is an old Microsoft two-button mouse with a scroll wheel that functions as buttons 3,4, and 5.  The trick is to define the ServerLayout as follows:

Section "ServerLayout"
	Identifier     "X.org Configured"
	Screen      0  "Screen0" 0 0
	InputDevice    "Mouse0" "CorePointer"
	InputDevice    "BackupMouse" "SendCoreEvents"
	InputDevice    "Keyboard0" "CoreKeyboard"
EndSection

Note that there can only be ONE “CorePointer” so set that to be your primary mouse.  Set the backup mouse to “SendCoreEvents” so that it receives mouse events, but X.org won’t freak out if it’s not present.