John W. Linville, from the CoCo Crew podcast, has published an article showing his design of an adapter that allows you to use SEGA Genesis (or Megadrive) with the CoCo3. Also, he mention that a dual-joypad version of this adapter was produced and built by his podcasting partner Neil Blanchard.
You can watch a quick demo in the video below and read all about it following the link at the bottom.
[youtube tc8-KmxaOXM nolink]
Link: Retro Thinker