The correct answer is: "The command seems to pause and hang, and the add file operation will run when the backup is complete."
From BOL: https://docs.microsoft.com/en-us/sql/t-sql/statements/backup-transact-sql?view=sql-server-2017
SQL Server uses an online backup process to allow a database backup while the database is still in use. During a backup, most operations are possible; for example, INSERT, UPDATE, or DELETE statements are allowed during a backup operation.
Operations that cannot run during a database or transaction log backup include:
File management operations such as the ALTER DATABASE statement with either the ADD FILE or REMOVE FILE options.
Shrink database or shrink file operations. This includes auto-shrink operations.
If a backup operation overlaps with a file-management or shrink operation, a conflict arises. Regardless of which of the conflicting operation began first, the second operation waits for the lock set by the first operation to time out (the time-out period is controlled by a session timeout setting). If the lock is released during the time-out period, the second operation continues. If the lock times out, the second operation fails.