1. Primary Datacenter Confluence uses Database A.
2. Database A used oracle high availability dataguard to Secondary Datacenter Database B.
3. Confluence Instance in Primary Datacenter and Confluence in Secondary Datacenter are dns'd in a failover situation
4. If Primary Datacenter is lost - automatically seemlessly fails over to Secondary Confluence Instance.
5. End User see's no downtime or knows they were directed to the failover environment.

We've tested this type of setup in our development environment.

When the Secondary Confluence Instance connects to Database B it sees the connections that are created by Oracle Dataguard - user Public as a cluster and shuts down.

If there is a way to get the secondary confluence instance to connect to database B, we'd be buying another unlimited commercial license.