How to Backup a Log-shipped Mirror Database without breaking log-shipping
It is not possible to take a backup of a secondary log-shipped DB without bringing it online.  I believe to take a backup with log shipping, the secondary DB has to be brought online WITH RECOVERY as Standby/Read-Only won't allow the DB to be backed up.

Other sources that confirm:

https://social.msdn.microsoft.com/Forums/sqlserver/en-US/41d80415-80bc-4821-85b1-762ce9ea72f1/how-to-backup-the-log-shipping-secondary-database?forum=sqldatabaseengine

Can I backup (Full) the Log-Shipping Secondary Database?