Thanks for your post, this is Andrew from BackupAssist support.
If you are only wanting to run backups from Monday-Saturday, then the best way to achieve what you are after is to setup two separate jobs. The first job is setup to run Monday, Wednesday and Friday to NAS1 (using the custom rotation scheme), and the second to run Tuesday, Thursday and Saturday to backup to NAS2.
The only other way to do this is via some scripting, but it would require you to store the backup in a staging area before moving it to the required location.
I hope this helps, but please let me know if you have any questions.