Merge two postgress databases with identical schemas
Unfortunately, there is no simple way to do this. Essentially you will need to write a script to go through one of the databases, extract the appropriate information, and then insert it into the other database, making sure to manage the new id numbers you get and updating the incoming information as needed. The complexity of your schema will determine how difficult this is to manage, but it is doable; pg_upgrade uses a version of this technique to handle re-mapping on-disk files inside the Postgres system catalogs. 

On a side note, this is one of the arguments against using surrogate primary keys. Had you used entirely natural unique keys, you would be able to simply dump one database out and load it into the other.