"Supposing I have a 2T external drive and a 1T SSD on my MBP, after using CCC, can I copy other files to the 2T drive?"
The size of the CCC backup will equal that of the source that is being backed up.
Generally, you DO NOT WANT TO copy other files to that backup. You want it to be "a clone" of the source -- nothing more, nothing less.
If your source volume is 1tb, and your backup volume is 2tb, my suggestion is this:
PARTITION the backup drive into two equally-sized partitions.
Use the FIRST partition for your CCC cloned backup.
Use the second partition for general file storage.
This will work very well. The only difference is when you connect the 2tb drive, you will see TWO volumes on the desktop (one representing the CCC cloned backup, and the other representing your "additional storage" drive.
Be sure that you give each volume an appropriate, easily-reconizable name.
Bear in mind that if you do things this way, the second partition IS NOT "backed up".
If the files on that volume are important to you, they must be backed up TO ANOTHER DRIVE, as well.