There can be only one default database connect string per agentparm.txt file. Users can run DB jobs against different databases by overriding the default connection string at the job level. Another option is to install a second system agent, configure it with its own db plugin and specify a different database connection string.
See the following url for overriding the connection string at the job level: https://docops.ca.com/ca-workload-automation-ae/11-4-2/en/reference/ae-job-information-language/jil-job-definitions/connect_string-attribute-specify-database-resource-location