After recently learning that the Atari 2600 Joystick ports can be configured to be either input or output, I was able to get the Atari to control a Nintendo NES without out any major modifications. In this case I configured one of the Atari joystick ports to be input and the other port to output.
In theory you should be able to connect the Atari 2600 to 8 separate devices at once, since there are 8 pins that you can control.
Possiblilties include:
Connect Atari to any other systems.
Sync 2 Atari 2600's to get 4 channels of audio.
Sync Atari w/ a NES to get 7 channels of audio.
Using Batsly Adams' algorithm that checks for the rising edge, can even use the Atari as a clock to sync multiple systems.