Multiple System Photo Libraries

No really because each time you changed libraries you would have to do a complete re-sync of entire library with the online iCloud library. That can take a lot of time depending on the size of the library.

If your library will fit easily on a 1 Tb drive with room to grow you might consider this mobile external SSD to carry along with your laptop: 1TB OWC Envoy Pro mini 

OWC is considered to be the premier 3rd party hardware supplier for Macs by most of the experienced users here.

As Old Toad already wrote you (a user) can have only one System Photo Library.

But you could try the following solution.

Your normal user will have his System Photo Library on the internal SSD, for this Library you select "Optimized Photos"

You would normally work with this user.

Then you create a new user "Backup User".

This user has the identical Apple User ID that you currently use.

For this user you have the System Photo Library on your external SSD and select "Original Photos".

You would have to login every few days, only then the Library on the external drive will get the latest updates.

You could run your Time Machine backups with this user, it would include the original photos.

I agree this is not the best solution, but it will combine what you want.

Regards

Ralf

Pretty clever, Ralf !