Can't set up mirroring for a specific database
Create full backup of the primary database
  Create backup of the transaction log, in the same backup file
  Restore that backup on the secondary location, with the norecovery option
  Set partner on mirror, then primary. 
  


When the mirroring session is established the primary must have the current LSN of the mirror-to-be in the active log. This is necessary so that the primary can start sending log records to the mirror to bring it up to date, w/o any interruption. 

In your sequence of events you have two LSNs saved: one after the full backup (LSN1), one after the log backup (LSN2). The primary will be able to accommodate a mirror that is at LSN2, but not one at LSN1. If you restore the full backup with no-recovery and then also restore the log, also with no-recovery, that should end up with a proper mirroring candidate database. But if you only restore the full backup then the mirror will be at LSN1 and the primary will reject it.