Can different user accounts have separate Time Machine Backup drives? If I have two users accounts on one Mac is it possible to automatically use a different Time Machine drive for each user's data?
No, because Time Machine works at the filesystem level of the whole disk, not just the files owned by any given user. In other words, Time Machine backs up the entire Macintosh HD, sans exclusions, and doesn't know or care about individual users from this perspective.

Have a look at: How Time Machine Works its Magic