That is what I had tried previously, however on occasion the job would fail after the first disk job was written to tape (locked both tapes which is exactly what I want, writing to both tapes at the same time to reduce backup window) however when the second job tried to write to the tape about 8 hours later than usual (due to Compacting kicking in when I have it scheduled for a Sunday, but actually starts on a Monday night).
It would appear that there is some sort of append timeout where the tape gets marked as unappendable.
Can you confirm if there are any mechanisms in place in Veeam 9 which would cause this?
All backups to tape work fine any other day. This is only an issue when Compacting kicks in between the disk and tape jobs, thus delaying writing to disk of the 2nd part of the tape job by anywhere between 2 and 10 hours.