Over the past month or so, I've spent some time with the old TGB Dual source code. After cleaning up the program (i.e. getting it to compile without errors), I translated it to English and added a couple new features:
  • the ability to load/save Goomba ROMs (GB emulator for GBA)
  • A "mirror mode" that flips the screen horizontally and switches left & right
  • Option to choose the extension for your save files

I also made the program in Unicode, so even the Japanese version will show up correctly on modern PCs.

Since I can't post URLs yet, I'll just say that the program is available on lakora.us (scroll down to TGB Dual under GBA Projects.)

P.S. Right next to it on the site is a version of the NRage input plugin that I worked on a couple months ago, to add the same Goomba ROM support. I also had to fix an unrelated piece of code to get Pokemon Yellow to work. If anyone has been able to get it working with older versions, could you try mine and let me know if my changes broke something on your end?