Each CPU can exchange data with the other using Message (MSG) instructions, or by using Produced/Consumed Tags.
The ControlLogix CPUs do not automatically have access to one another's tag databases, like you might find in a distributed control system.
I recommend the EtherNet/IP Network Configuration Manual, Rockwell Automation publication number ENET-UM001, available in PDF format from the RA Literature Library server. Chapter 7 describes the basics of both Messaging and Produced/Consumed Tags.