Merging two backups form a postgresql database
I don't know an easy way, but I'd try this:


Restore both databases individually and dump out only the data using INSERT statements (instead of COPY).  Something like pg_dump -a --inserts.
Restore the schema from the original backup into the target database.
Disable all foreign key constraints.
Restore the first dump created in 1.
Restore the second dump created in 1., ignore errors.
Enable foreign key constraints.