Make encrypted differential backup to same media
As Aaron noted, there's no reason to retain multiple differential backups, as you would only use the latest, and only as an optimization.

The limitation on one encrypted backup per file is in the main doc page for Encrypted Backups:


  The following are restrictions that apply to the encryption options: 
  
  . . .
  
  Appending to an existing backup set option is not supported for
  encrypted backups.


Backup Encryption