When you say "game link cable" most people think of Nintendo's Gameboy, which, from the very first incarnation in 1989, featured a game link cable which allowed people to play each other each using their own Gameboy.
But before the Gameboy, there was Atari ST, a home computer that came with MIDI ports (Musical Intrument Digital Interface) which allowed the computer to record or control musical intstruments. In 1987 Atari released Midi Maze which used the MIDI cable to communicate game state to another Atari ST, allowing for a multiplayer game. Midi Maze received moderate reviews but was hailed as revolutionary for connecting home computers. It is considered the first deathmatch game and listed as one of the ten most influential games in first person shooters.
But before the Atari ST, there was Computer Mahjong Yakuman. Released in 1983, this was a handheld electronic game that played the tile-based strategy game Mahjong and featured a game link cable allowing two people to play each other. It had poor adoption but a huge legacy. One of its creators, Satoru Okada, became assistant director of the team developing the first GameBoy, and insisted GameBoy must support a game link cable despite pushback from the technical team.
But before Computer Mahjong Yakuman, there was the Atari 2600, and it also was game linked... in the lab.
In 1979, Warren Robinett was making Atari Adventure, and was looking for a way to make the game more engaging. Years before Gameboy or MidiMaze or Yakuman he theorized he could support a multi-player version by linking consoles. He prototyped a cable that ran from the 2-player joystick port of one Atari 2600 to the 2-player joystick port of another, and was able to sync data between the consoles. However, his relationship with Atari management was so bad he abandoned the plan, released Atari Adventure as is, and quit the company.
Just take a moment to imagine what might have happened if Atari management hadn't blown it and allowed the game to be released with a game link cable. Imagine what other Atari games might have supported that cable. Defender? Berserk? Haunted House? Imagine how Atari could have leveraged its market dominance. Imagine if Nintendo and Sega had felt they had to follow. We would have had VS mode for Super Mario World a decade earlier. How would this have changed video games today?
Unfortunately, the only place this is documented is in Warren Robinett's book about the making of Adventure, which is yet to be published. However, there are emails from Robinett confirming the story.
I got to meet Warren Robinett because Ernest Cline dragged him to a book signing. I got to thank him for an awesome game and relate my only complaint - the lack of multiplayer. That's when he told me the above story and blew my mind. This guy was amazing! I had already started dabbling with porting Adventure to modern systems, but then I knew I had a calling. I wanted to play that version he was imagining. And Head-to-Head Atari Adventure was born. And it is a blast to play!
I claim this is a pixel perfect reproduction of the original game, but there are a couple discrepancies. Robinett's vision would have only supported a second player, but I added a possible third. Obviously I had to add more castles. I had to rework the randomization algorithm (I am amazed he was able to pull off the original with so little code - but it fell apart as you added more keys and castles). And I changed a couple details for better multiplayer gameplay (e.g. the walls in catacombs do not hide objects from view and keys cannot be embedded in the walls inside home castles).
I hope you enjoy Warren's game. h2hadventure.com