you need to shutdown Wildfire to do this. There is the bin/extra/embedded-db-viewer.sh which allows you to access the database and execute SQL if it is down. The embedded mode does not allow to connect via JDBC or to connect with two clients.
You could also write a Wildfire plugin which uses Wildfire’‘s connection pool to run some queries but I guess it’‘s more easy to use another standalone database. One can of course run HSQLDB as a standalone database and use JDBC to connect to it. But that’'s not the simple out-of-the-box Wildifre setup.