Why not just look at what your Amiga mouse does... which has two buttons.
Try the mouse int he game port and try fire 2.
I haven’t looked right into it, but this diagram looks like the second button is active high
(ie. it does have to be pulled high to fire).
which is different than the directions and fire 1.
In the diagram, the backslash prefix means the signal is active low.
http://3.bp.blogspot.com/-chCCJ6kNy7...34.18%2BAM.png