This game has a two player link mode. Only one cartridge is needed. Make sure the master end of the link cable is plugged into the GBA with the cartridge. Chose "2 player" from the main menu. Don't turn on the second GBA until instructed to do so.
I don't have a wii u anymore to test it myself, but if homebrews are not visible on the wii u menu I think you can press L + R + minus to open the plugin menu, there should be an option called "homebrews on wii u menu" or something similar
Or when your player dies, you can say in code, if enemy exists, do nothing, but if enemy does not exist, then create enemy at certain spot. (This would be a pain tho for lots of emeies)