Time Machine: backing up 2 drives at once

[Numo]

Recently I made an external SSD my startup disk, so I am now using my iMac internal HD for storage. But I can't find out how to get TM to backup both of these disks at the same time.

I assume TM automatically backs up the boot disk. Is it possible to have TM include my internal drive when it backs up the external SSD?

I know I could back up the internal with CCC but that is an extra step and not automated.