queste sono le sezioni che devi modificare:

Section "InputDevice"
Identifier "Mouse2"
Driver "mouse"
Option "Protocol" "PS/2"
Option "Device" "/dev/psaux"
Option "Emulate3Buttons"
Option "Emulate3Timeout" "50"
EndSection

Section "ServerLayout"
InputDevice "Mouse1" "CorePointer"
InputDevice "Mouse2" "SendCoreEvents"