well, you need a communication link, which would mean one of the following:
- wires/fiber optic cable connecting them
- 2 way wireless communication (IE radio transceivers, lasers/diodes, etc)
- haptic, smell, sound (not really viable solutions, but they are other ways things could communicate)
Given your goal, you probably want some sort of 2 way wireless communication system... an easy solution could be to use a wifi adapter on each and setup a network that they can use to talk to each other. Your going to need a low latancy reasonable bandidth link, as they are going to need to communicate state information with each other at a reasonable rate with minimal delay.