- How can I merge the two databases with two different files?
It's done by importing all entries from database B into database A.

1. open database B in OrangeCD Catalog
2. click menu Database -> Manage -> Import and select database B as source.
3. on next page, leave "All albums" selected.

Since this action cannot be undone, I recommend making backups of both databases in case something goes wrong (menu Database -> Backup).

- Do I need to register twice and pay twice or can I use the same s/n on both locations?
You can use same number in this scenario.

Andrei