If you define any other pools, you will also need to add an ns/server/yourserver/acs/database section to your config file with an AvailablePool entry for each ACS database pool (pool1, pool2, pool3). If no such entries exist the DB API will use all available pools (including the one going to your other database).
One way of being able to use the DB API in your queries against this other data would be to have a scheduled proc. accessing the external data with the ns_db API and copying changed data to a set of local tables. This way you could use knowledge of how the data changes to only copy across what is required at each update, and run the synchroniser more frequently than would be possible if you were doing a full dump & copy...
