What version of controller? If they are older robots, you'll need the space check software option, on newer robots it's called interference check. There are two levels of interference check, basic and intelligent. Unless your robots are on an RTU, you will probably be okay with the basic version.
There are other ways to setup interlocks, such as using reference positions, dcs zones, and just basic I/O handshakes, but none of those is as fool-proof as space check. With space check all you need to do is setup the zones where the robots could collide with each other. That's it. With any other method you'll have to have absolutely bulletproof code that has been tested through all permutations of how an operator could try and start the system.
Space/interference check isn't cheap, but it saves hours of program setup and testing, plus, it just works.