Ok, is it correct that your problem is that tar does not know how to flip the disk?
In that case, use "star". That is a more or less upward compatible "tar". You can get it
here
Star offers you an option where you can specify a script name
(new-volume-script=
script)when the media has to be changed. Since you know what command (mtx etc) you have to issue for changing the disc, you can specify that.
I also wrote
this post about star in this forum.
BTW, are you sure that you want to make multi volume backups? If one media fails, you loose everything! On my system, I made all my disk partitions equal to the size of my backup media. So a partition is guaranteed to fit on my media. Restore and verify is MUCH easier and more reliable that way.
Let me know if this was your problem and whether you could solve it.
jlinkels