There are a number of games that can be played using serial null modem link ups on Amiga and Atari ST computers. For Stunt Car Racer (and possibly others) you can link an Amiga to an Atari ST for multiplayer.
To do this all that is required is a Null Modem cable. These can be bought, and can also be made very easily. There is no difference in the cable to connect 2 x Amiga, 2 x Atari ST or Amiga to Atari ST. The pin out for the cable is below. It simply requires 2 x female DB25 connectors, wired as follows. For reference I have also included the Amiga serial cable pin out.
Null modem pin out from http://www.amigaforever.com/kb/13-105
Amiga Serial pin out from http://www.allpinouts.org/index.php/Amiga_Serial_25_pin
So below is one side wired up. Ignore the spare brown wire on the right.
Connecting the Amiga & Atari ST computers together couldn’t be simpler. Just connect the serial cable in both computers. When you start Stunt Car Racer, select the link up option on one of the computers, then select the link option on the other and it will connect almost instantly. Names and track selection are entered on the computer which kicked off the link up option first, then you just need a mate to come round so you can get into racing!