I am doing the code for a 2nd DB9 Connector for the Retro Adapter. It converts the DB15 into another DB9 so you can connect two controllers simultaneously for 2 players.
Atari/Commodore/Sega controllers are working and I will add support for NES and SNES too. I will probably do N64 as well. The button mapping is not ideal but is usable. I experimented with having the RA re-enumerate as two separate controllers but it is not ideal to do on the fly. The current system is a good compromise.