Database schema is not related to ATG but related to database.
Its the database user where we create the table and store the table.
ATG uses this database schema to work interact with database like any other application.
So you can create more than one XXXRepository files with difference values of JNDIName properties. each XXXRepotitory file will contain info. about difference datasource. So you can have multiple datasource with your application.
In your server, datasource xml file have these JNDI names registered with all mantatory entries like (username, password, url etc).
If someone have better idea. please share.
You can learn in here http://docs.oracle.com/cd/E23095_01/Platform.93/RepositoryGuide/html/index.html the repository guide will tallk you about the different schemas that atg uses for catalog, commerce and merchindising, and It does have informatio about how you manipulate the database from atg stand point.