Very similar databases

If you have two very similar databases (one on your laptop and one on your workstation), then the easiest method is probably ediff. Use M-x ediff to compare the two files.

M-x bbdb-merge-file does this for you.

Very different databases

If you want to merge two very different databases (one from your girlfriend, one from yourself), your best bet is to just concatenate the two. Then start the BBDB, and use M-x bbdb-show-duplicates. For every record that appears twice, put point on the second record, and use ‘r’ to merge it into the first record.