The server services layou you've choosen is a bit problematic as mirroring works for all services configured of a server. So when you choose to mirror the system, then web, db and dns gets mirrored from server a to server b and you loose the ability to create mail records. So mirroring is most liekly not a option when you dont want to reinstall everything and choose a different services layout.
The only option that remains is to not use automatic mirroring and use bind replication instead. This means that you have to create a primary zone record for the first server and a secondary zone record for the second server and let bind do the replication of the zone content.