When using NetBackup for SQL Server, a multiplexed backup won't restore without de-multiplexing the images.

When restoring from a NetBackup for SQL Server multiplexed backup, the device count must be the same as, or higher, than the stream count, or the restore may fail.  Microsoft SQL Server requires all restore streams be available before it creates the destination database files.  If the stream count exceeds the number of devices, the restore may fail, regardless of the client read timeout setting.  If the images are de-multiplexed, however, the restore is successful.  

More recent versions of the VERITAS NetBackup for Microsoft SQL Server System Administrator's Guide for Windows states:
VERITAS does not recommend enabling multiplexing for a schedule used for backing up multiple stripes.  When restoring from a multi-stream backup, all backup images must be available simultaneously.

If a SQL Server multiplexed backup wrote more than one stream image (SQL stripe) to a single tape and fails to restore, all but 1 of the stream images must be de-multiplexed (using bpduplicate ) to its own tape or disk image for successful restore. Future backups should be reconfigured to use a single tape per stream (stripe) to prevent the need for this workaround.

Refer to the related article (s) on how to de-multiplex a SQL Server backup image, found below, in the "Related Documents" section.