Backing up external hard drive shared by two macs to network share without duplication
Another solution is don't use time machine for this type of backup.

I use Chronosync (a GUI for sync) and in this case the scheduler would just skip the sync if the volume is not available. If it is available it would sync the drive to the NAS. You could also use Carbon Copy Cloner for this function.

If you feel comfortable with the terminal and you don't want to pay the money for Chronosync or its ilk, then rsync and chron could also be used.

Good luck!