A label already defined error means that you added something that has the same name as something else that already exists elsewhere in the code.
Using multiple banks to store music sounds potentially like a bottomless pit problems getting it to play nice with NESmaker. I know only a little bit of how the sound engine works currently, but I suspect that even if you got bank switch music to work in engine, every time you wanted to revise and import your tracks you would have to first import the ones for the second bank, probably export to generate the asm files, go into the code, cut the generated code out of the first bank where it defaults to and paste it into the second bank, and lastly import the music for the first bank. Then you would have to deal with NESmaker only having references to the music in the first bank when assigning music to screens. You would have to go into the code every time you export your game and manually adjust what song from the second bank each screen is playing.
If this still sounds like something you want to do, the first thing I would try to figure out is how does a track get loaded. Then I would want to know if it looks like a bank switch can happen there. And lastly I would want to know if the sound data and the ggsound engine can exist on different banks. If they can't then you might need to have a copy of ggsound in each bank and somehow tell the game engine which ggsound it should be playing when it handles music.
I don't know, if it was me who wanted to do these kind of changes in the current version of NESmaker and the complexity of the music was like the most important aspect to the game, I would probably look towards making this game for a more modern engine instead of the NES at that point.