I am working in a project with two UR robot feeding different CNC machine. What is the way of communicating or connect those two UR robots not crash to each other and to work properly ? what is the easy way to connect to each other? I highly appreciated for the help. 

We have used modbus to communicate between multiple robots having each of the robots submitting data to registers and then the other robots could read the data and make decisions based on the data.  This could be used to prevent two robots from being in the same area at once, there are race conditions that need to be monitored such that both robots want to enter the same area and neither is there currently, how do you prevent them from both making the decision to enter the area at the same time and then colliding?  This can be accomplished by making one of the robots the master and the other the slave. So that only one of them can make the decision on which robot is to enter the shared space at any given time.  Generally though, we have conditions that they both must meet to enter the area and we have set-up the cells so that only one robots conditions can be satisfied at any given moment.

We have also used Profinet over Siemens PLC to control two robots, we do this when we need a lot of coordination between the actions of the robots, not just ensuring they do not enter the same space due to working in close proximity.

The other option, if the robots are just close to each other but do not actually need to share a space, is you could just use joint limits or safety planes to ensure that the two robots never enter the space where they could collide with each other