At the moment our supported platforms doc does not specify if we support clustered versions of supported databases, eg. Oracle RAC, MySQL Cluster,MSSQL Cluster & etc.
While they might work with Confluence, we do not specifically test against them.
Supporting clustered databases can be an attractive feature for Enterprise customers.
While this functionality is not currently guaranteed nor supported, if MS SQL is clusterd, you can try this format to see if it will work with your environment.