Tags and notes hands are stored in the database they are created on and you won't get them just by reimporting hand history files into another database. You can export/import manual hand/player notes via 'Database -> Notes' but if you import notes into another database then those notes are added to the existing notes in that database and if the same note exists in both databases then the note text is appended meaning that it will be doubled up so this isn't ideal if you are going to import notes from one database into another and then back again. You could delete notes before importing but there isn't a built in method to purge all manual notes so you'll need to use PGAdmin (which is in Start -> All Programs -> PostgreSQL) and run the following query:
TRUNCATE TABLE notes;
To do that double click on PostgreSQL x.y' (where x.y is your PostgreSQL version) and if your prompted for a password the default is "dbpass" (without the quotes), expand 'Databases' and then select your database. The query tool can be opened via its icon in PGAdmin or via 'Tools -> Query Tool' and then you can run the query using 'Query -> Execute'. I'd recommend backing up your poker data before making any changes since PGAdmin is connected directly to your database so take care not to make any other unintended changes to your data while using it.
Tags can only be directly transferred from one computer to another via a
backup & restore of the entire database but it's not possible to merge the tags in one database with the tags in another. Also if one computer is always on then another option so you don't have to manually sync is to have the second machine sharing the same PostgreSQL/database:
Guide: PostgreSQL Server Hosted on a Local Network