Is it possible to add the 2nd fire function to do the same thing as UP?
Wow I didn't realise the C64 supported a second button ! I haven't had a chance to work on this lately, but if i ever get back to it will investigate adding this.
Some discussion on games that support secondary button here:
https://retrocomputing.stackexchange.com/questions/2499/which-c64-games-support-...
Now there are tons of games with second fire: http://wiki.icomp.de/wiki/DE-9_Joystick#C64
and look at this thread; https://www.lemon64.com/forum/viewtopic.php?p=970366