Thanks for interesting question. Per QA VMs are processed in alphabetic order.
I would hope to ensure that all big jobs are written to tape first (using both tapes) assuming that the remaining smaller jobs will better fill the tape.
For tape parallel processing specific logic is used. We have check free space remaining on the last tape media in every media set and pick the tape with that has enough free space to store the backup. Aside from that full backup files are written on the first place, then VBR starts to write the incremental backups.