Using Time Machine with 2 internal drives My sister's 2012 MacBook is running terribly slow, so I adviced her to install an ssd. Since there are budget constraints, the best option would be to install de HDD in the optical bay place for storage and a 240gb ssd as system drive.

The thing is, she likes to constantly back up his system in an external drive using Time Machine and there's where my doubt raises.

Will time machine save only one drive? Can it save both drives and all their configurations as a single image?

I really want to help her saving some money

Thanks for your time :)
I agree that TimeMachine will backup both internal volumes.
https://www.baligu.com/pondini/TM/32.html

My sister's 2012 MacBook is running terribly slow

I would spend a little time diagnosing why the machine is running slow. 


Run activity monitor /Applications/Utilities/Activity Monitor.  Check ram usage and paging.
Run etrecheck https://itunes.apple.com/us/app/etrecheck/id1423715984?mt=12