How do I make the middle touch pad's button work?

xinput --test 11

yields

button press   1 
button press   1 

For pressing both the left and the middle button.

I have tried to do

xinput set-button-map 11 1 4 2

and so on, but as the --test shows that button 1 is being depressed, then probably the issue is at a lower level than with X11's perception of what mouse buttons I'm pressing (or assigning button-map 11 1 2 3 and clicking the right button in firefox, wouldn't trigger the middle-click on the link)