So I have run into this problem a couple of times when running a job at the end of a month (when my GFS job fails, which is a different issue) where there are multiple source jobs and parallel processing is checked. It works great in terms of speed since it uses both drives, but it ends up leaving me with two tapes less than half full because the job doesn't appear to do any sort of pre-process layout planning.
Veeam already knows when it goes to pull the .vbk files to tape how large each of them are, It even calculates how much space is used by all of the fulls combined in the job creation wizard. Would it be possible to add some kind of logic that checks to see if the data that one drive is about to put on a new tape would fit on whatever space is left after the task working on another tape is finished?
I just exported two LTO5 (uncompressed) tapes that have 1TB of free space each for this month. It doesn't impact me that much as we have saved considerably on tape usage since moving to Veeam but like many organizations our off-site storage space is limited and every little bit helps.
Veeam Certified Architect