MacOS – Will Time Machine store the same data twice in this situation

Seeing that external storage is relatively inexpensive, I would leave the existing TM drive alone and purchase another external drive. That way, you can backup the additional data on the new drive to the TM disk, too. You wouldn't be able to do this if the additional data resides on the TM drive itself.

If you do choose to use the TM disk for non-TM data, I recommend partitioning the drive to separate the TM data from the non-TM data. If you ever need to reformat the TM partition, you won't need to move that additional data to another disk first, or visa versa.

All that being said, it is technically possible to use the TM disk to store additional data. I used to do it until #4 (below) happened and I lost ~10k .mp3 files I then needed to re-rip from CDs. (I did re-rip about 80% of the lost files to Apple Lossless format, so it wasn't a complete disaster, but I did lose thousands of files.)

There are several drawbacks you should keep in mind:

1. Time Machine will delete older backups more frequently when available space is low.
2. Performance on the drive will suffer when TM is running and you are accessing the additional data at the same time.
3. If you need to transport the additional data off-site, you'll also be taking your TM backup with you.
4. When (not if) the entire disk crashes hard, you will lose the additional data unless it is backed up somewhere.

The short answer is no, but here is why and how the exclusions work when you pick different rules for two accounts.

Whether you have one backup destination or two, the rules for multiple users is to combine the exclusions for each user with the exclusions from the system so that each user can exclude different things, but there is no "inclusion" mechanism to add something back that another user has denied.

There is a nice article on having multiple users and goes into more details on setting up multiple exclusion lists:

• http://pondini.org/TM/10.html

It's not going to save backup time or space as you asked, but perhaps drives can get larger and you could just have both drives hold the same content since incremental updates mean that unless a file changes, it only gets copied once to each destination, speeding backups for subsequent backups to each individual drive when it's connected.

Pulling one drive from rotation with two works well as the system just carries on with the drive connected and then when the further out of date drive mounts again, one of the next two backups will catch it up with all changes since it was last mounted during a backup.