If I have a master node 1 (with database A) and a master node 2 (with database B) can I use repmgr and have both A and B replicated to a hot standby node 3 ?
[ EDIT: They would all be running Postgres 9.3 ]
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.