Hi,
When you connect(rejoin) the backup device to the cluster successfully , after they settle down with one priamary and one secondary status.. just issue a commit from the node (where you have made changes) , it should automatically synchronize the configuration between the nodes.
Ideally , in a cluster setup , once the configuration is synchronized between the nodes, for further changes , you can issue the commit from any node (primary or secondary) .
Hope this answers your question !!