repmgr: Can I replicate databases from two different masters to a third slave?
PostgreSQL's built-in replication (as used by repmgr) replicates entire PostgreSQL instances - with all databases. The transaction log is shared between all databases.

PostgreSQL can't merge transaction logs from another instance. Nor can it replicate only a subset of databases. You have to replicate exactly one instance to exactly one other instance.

If you want something other than that, there are a variety of other replication options such as Londiste, Slony-I, BDR, etc.