I don't know how it looks like on VLS9000 (OEM of Sepaton) .. But it's possible on different hardware (CentricStor).
If you have 2 VTLs working with mirror (replication) probably 2 boxes are parts of a VTL cluster. NB see only one VTL (robot, ACSLS) .. If site A goes down VTL server starts at site B. NB can access robot in site B. Access to robot is provided only from one site, in other words there is no ACSLS server working in site A and site B at the same time. If you have 2 boxes with mirrored storage you can access a tape in 2 sites but no at the same time. You have only one robot so Nb will not let you put one tape to two tape drives.