I had the situation once, Fanuc says for IIC between the 2 robots to stop collision on a single rail, they have to be oriented in the same directions (ie their X directions must be the same). Mine were facing each other.
They couldn't help me at all and while waiting for their help, I got an epiphany. I apologize now for the very quick representation of what I did.
I created bubble zones under each robot, 10,000mm below (-Z), large enough to cover the reach of each robot.
Link number was ZERO, Worked very well, and covers even manual jogging. Hope this can help you.