NO$GBA started as a Gameboy Advance debugger but was later released as a normal emulator(version 1.9). It is the first gameboy advance emulator to support multiplayer games on the same pc! Later versions added support for the NintendoDS portable console.
How to use multiplayer
put the GBA BIOS file in the emulator's folder, and rename it to "gba.rom"
pick a GBA image (no ZIP)
go to "Options" / "Emulation setup"
increase the number of "Emulated Gameboys"
in "Link Gamepaks", set "Gamepaks in all GBA"
in "Reset/Startup Entrypoint", set "GBA BIOS (Nintendo logo")
click OK, you should then see 2 GBA screens side by side
right click on the left window, click on "Link" so that the window comes back to normal.
then redo the right-click and click "Link" , and here we go, the 2nd gameboy does boot!
Please remember that the more gameboy's you emulate the stonger the pc will have to be!