It certainly was possible to have multiple active concurrent restores, for multiple different clients, from the multi-plexed backups from the same tape... IF... you start the restores within around 180 seconds of each other (i.e. the whole set). Well, it definitely used to work in NetBackup v6.5 and so I would still expect it to work. And I see no reason why it shouldn't work for multi-plexed restore from multi-plexed backups from the same client.
It just takes careful planning and detailed testing.