Ya, I skimmed through that guide. Other than it being an older version, with possibly different procedures than the latest LTS, it appeared to be a solution for creating groups or clusters. This requires an odd number of servers to maintain cluster quorum (unless you can fake it with a share witness on server 1 or 2). That’s great for organizations the need/want the ability to scale out like that. For smaller organizations though, I think this might be a better fit.
After some sleep I thought, it may be possible to take an existing single node, setup master-slave replication from existing node to new node, then once replication is complete, setup the other replication link from new node to existing node to complete the master-master replication. However, PrivacyIDEA is my first dive into MySQL, or any DB application really, so I could be completely wrong.