That is on the more ambitious side for a newcomer hack (no way around getting your hands dirty with some assembly really unless you do some really elaborate cheat which is probably beyond what the likes of http://bsfree.org/hack/hacking_gb.html
will afford you) but still well within reason.
Both equips probably have a flag in memory to say they are equipped or maybe will have a this item only works with this type setup.
You would just change the if/compare setup to see if the thing is equipped to is the thing in your inventory instead. Changing it to collisions could be harder though.
Double tapping is a bit harder as you will have to add something to check the directions and then note it and do something upon the second hit. Nothing too drastic (read it, note it, wait a bit, check again, reset if not or run if so).
Finding the code you want to be looking at is potentially on the trickier side. I normally don't like to watch things that watch buttons but it is still a valid path. You could try equipped vs not equipped and finding the location of the equipped item (and/or any effects elsewhere) and seeing if something checks that (the select a slot making it slightly harder).