Hello @Paul Hutchings
Thanks for the great question, it is going to be a hard one to achieve on a dynamic level inside the same Aux copy job. The Aux copy job settings around amount of streams and drives to use are set at the beginning of the job and cannot be changed on the fly.
I would recommend killing the active Aux copy job when backup jobs are scheduled to run, submitted a new Aux copy job to run that has the setting to only use the one tape drive and then revert this back once the backup jobs are complete.
The only way to automate this process would be with scheduling and max run time. If you know how long your backup jobs run for you could make it so the Aux copy job kills it self before they start and a new schedule with the new drive settings takes over and then kills its self after X amount of hours. After this the original schedule with 2 drives takes over.
The above process is very messing and overly complex for a simple issue. If this Aux copy data is a one off backlog i would not bother with this and just simply resubmit the job when you are in your backup window.
Please note that killing a Aux copy job has no effect on the data it has processed to that point, it will check point and the next job running will pick up where it left off.
Kind regards
Albert Williams