By default Spring boot does a lot for you in datasource configuration, but if you’re using two separate database types DB2, mssql, mysql, Oracle, etc. It doesn’t know how to infer which database type each is, so if you are configuring multiple different database types in Spring you need to specify the jpa.database type. In a yaml configuration your datasources should be defines like so.
spring: datasource: url: jdbc:sqlserver://${server}:${port};databaseName=${databaseName} username: ${username} password: ${password} jpa: database: sql_server db2: datasource: jdbcurl: jdbc:db2://${server}:${port}/${databaseName} username: ${username} password: ${password} jpa: database: db2