Time Machine backups (2 backups at same time) I have 2 questions regarding time machine backups:

I have a touch bar MacBook Pro 15'' with 256GB SSD (let call it A) and macOS 10.14.3. Currently I perform a Time Machine backup on an external 1TB HD (let call it B). I use Photos with an external 2TB HD (let call it C) in order to maintain the library on an external HD.

1) It is possible, with Time Machine, to backup an external HD into another external HD? I want to backup the Photo library of the HD (C) into an HD (D) (C and D are same size: 2TB) using Time Machine.

2) It is possible to backup A into the external disk B with Time Machine and also, at the same time, backup C into D with the same Time Machine?
By default, time machine excludes external drives that are mounted, but you can add them back into your backup in Time Machine "options" (found on the Time Machine Prefrence Pane). See here for more info: https://www.macworld.com/article/3153995/how-to-make-sure-time-machine-backs-up-external-drives.html

To add an external drive into the list of Time Machine Backup Drives, just use the "Add Time Machine Backup Disk" in the Time Machine preference pane.

Regarding your Question (2), You can not direct a some of your backup to one time machine, and the rest to another, thought that would be a nice feature. I personally use an rsync script that runs nightly to solve a similar problem, though that's nowhere near as convenient (or user-friendly) as Time Machine.