$ man mousedrv | grep -A6 Emulate3Buttons
       Option "Emulate3Buttons" "boolean"
              Enable/disable the emulation of the third (middle) mouse  button
              for mice which only have two physical buttons.  The third button
              is emulated by pressing both buttons  simultaneously.   Default:
              on, until a press of a physical button 3 is detected.  Property:
              "Mouse Middle Button Emulation"