It seems to me that you need to write some PLC relations so that the robots are talking to each other through the plc. You will need to find open DI's in each robot, write them down. Then you need to find the appropriate DO's form your robots that are signaling that they are clear. Create a DO from the plc to each robot that turns on when it receives the DO from the other robot. This signal will be your new robot DI. The signal will probably be sent through your LAN connection.
Also you will need to add some kind of wait command that correlates to your new DI where you are having your collisions.
I hope this makes sense to you. You need to use the plc like a relay but right now it seems like you only are sending a message out, but not relaying it to the other robot.