In my one of assignment, requirement was to support multiple datasources for the application. I searched through the web for the same and found that Grails 2.X.X support the multiple datasources out of the box. You just have to configure it in your DataSource.groovy and code looks like:

 development {
        dataSource {
            dbCreate = "create-drop" // one of 'create', 'create-drop', 'update', 'validate', ''
            url = "jdbc:h2:mem:devDb;MVCC=TRUE;LOCK_TIMEOUT=10000"
        }
        dataSource_master {
            dbCreate = "create-drop" // one of 'create', 'create-drop', 'update', 'validate', ''
            url = "jdbc:h2:mem:masterDb;MVCC=TRUE;LOCK_TIMEOUT=10000"
        }
        dataSource_clientname1 {
            dbCreate = "create-drop" // one of 'create', 'create-drop', 'update', 'validate', ''
            url = "jdbc:h2:mem:com1Db;MVCC=TRUE;LOCK_TIMEOUT=10000"
        }
        dataSource_clientname2 {
            dbCreate = "create-drop" // one of 'create', 'create-drop', 'update', 'validate', ''
            url = "jdbc:h2:mem:com2Db;MVCC=TRUE;LOCK_TIMEOUT=10000"
        }
    }

Note : All datasoure name must have the prefix “dataSource_”.

Hope this help.