You get the below error on the second backup

Msg 3023, Level 16, State 2, Line 1

Backup, file manipulation operations (such as ALTER DATABASE ADD FILE) and encryption changes on a database must be serialized. Reissue the statement after the current backup or file manipulation operation is completed.

Msg 3204, Level 16, State 1, Line 1

The backup or restore was aborted.

Msg 3013, Level 16, State 1, Line 1

BACKUP DATABASE is terminating abnormally.