Why is SnowMirror using two databases?
The main database is the mirror DB where the ServiceNow data is being stored. However SnowMirror requires to maintain its configuration in a second database. So called config DB. The config DB is configured during the installation phase and it is using H2 embedded database by default. The mirror DB can be configured in SnowMirror Settings page.
See a picture of the whole SnowMirror architecture.