If you want the pairs to be separate from every other pair then each pair will need a separate address (or maybe a pair of addresses, depending on how you plan to write the programs). (green text added)
One option might be to add an 8-way DIP switch to each unit and use it to select any of 256 pre-programmed addresses.
Another option would be to have a special "master" unit that communicates with a pair of new devices (assuming a new Tx and a new Rx have standard addresses that are known to the master and separate from each other). Then the master could send a message to each of them in turn with a new address for the pair. The master could maintain a "database" with all the addresses already issued so it would not issue duplicates. The Tx and the Rx Arduinos would need to store the new address in EEPROM memory.
This process is risky because the Tx or Rx might get the wrong address and neither the master nor its buddy could tak to it so it would be a good idea to have some means to get back to the "factory" settings.