You mentioned serial number is right, so perhaps you already checked this, but I recommend double-double checking on it again.
(Well, device configuration wizard should do this properly anyway ... but it's failing now, we better check)

As you might know already, NetBackup keeps robotic drive number and use it to mount tapes.

You have 6 tape drives in your second VTL, so the drives will be of 1 2 3 4 5 6, in your tpconfig -d output - probably with TLD(1), if your TLD(0) is the first VTL.

If the 1st and 2nd and 3rd tape drives in the VTL are hooked up to the seconf filer's nrst2a nrst0a nrst1a, then the tpconfig -d output should reflect the order right, with associated drive path.

filer2:nrst2a being DRIVE = 1 in TLD(1)
filer2:nrst0a being DRIVE = 2 in TLD(1)
filer2:nrst1a being DRIVE = 3 in TLD(1)

And then if the 4th 5th 6th drives are hooked up to the first filer's nrst3a nrst5a nrst 6a, ,then tpconfig -d should be like this.

filer2:nrst2a being DRIVE = 1 in TLD(1)
filer2:nrst0a being DRIVE = 2 in TLD(1)
filer2:nrst1a being DRIVE = 3 in TLD(1)
filer1:nrst3a being DRIVE = 4 in TLD(1)
filer1:nrst5a being DRIVE = 5 in TLD(1)
filer1:nrst6a being DRIVE = 6 in TLD(1)

One good way to verify if this mapping is right or wrong is to run /usr/openv/volmgr/bin/robtest and move "one tape" at a time, from slot 1 to drive 1, then to drive 2, then to drive 3 ... in TLD(1) while running "mt -f status" syntax from the filer's command prompt.

Abe