Follow the directions for adding licenses into the Primary RTM. The steps are identical.
Only the licenses in the Primary RTM need to be activated.
Once you have installed and started the Secondary RTM, it connects to the Primary RTM and retrieves the licensing information but does not activate the licenses until the Primary fails. See the diagram below
Read about activating custom licenses.
Warning: Do not activate both the Primary and Secondary RTM licenses concurrently. Doing so may cause your licenses to split between the two RTM's, forcing backup on only part of your licenses.