Question

Is it possible to mirror two sql server of same group?

Answer 1

You are correct, it is impossible to mirror a database that is part of an Always On Availability group. You can consider logshipping.

Ref:

https://blogs.msdn.microsoft.com/sqlalwayson/2010/12/08/alwayson-availability-group-and-database-mirroring/

"Does AlwaysOn Availability Group replace Database Mirroring going forward?” The short answer is we recommend that you migrate from the mirroring configuration or even mirroring and log shipping configuration to using Availability Group. Database Mirroring will still be available in the Denali release but will be phased out over subsequent releases. Log Shipping will continue to be available in future releases.

For the error you are getting read this document especially section: Windows System Requirements and Recommendations.

https://docs.microsoft.com/en-us/sql/database-engine/availability-groups/windows/prereqs-restrictions-recommendations-always-on-availability#PrerequisitesForAGs