TGB Dual is an open source (GPLv2) GB/GBC emulator with game link cable support.

This is a port of it to libretro.  To emulate two units side by side, use
your frontend's "Sufami Turbo" interface, placing the ROMs in "slots" A and B.
The Sufami Turbo BIOS itself can be any file; it is ignored by the emulator.
Use the Player 1 and Player 2 controllers to control each unit.

In addition, some of the Japanese comments in the source code have been run
through Google Translate to English, and CP932 and EUC-JP encoded files have
been converted to UTF-8.

Original sources from
http://gigo.retrogames.com/download.html#tgb-dual
http://shinh.skr.jp/tgbdualsdl/