In my own experience with backup software it's always been a requirement that the destination [backup] drive is as large as or larger than the source drive. I looked through the CCC manual but I didn't see anything specific saying what you want to do is possible -- however you might want to go directly to CCC/Bombich help with that question, for the most reliable reply..
I should point out that I always do a straight clone backup, not an archival backup [that would need even more extra space on the destination drive for older versions of files], so that's my point of reference..
I guess you could partition the larger [source] drive into a less-than 14TB partition and a 2nd partition for the rest, and then make partitions on the backup drives that match those sizes. If that worked then you might be able to use the remaining space on the 2nd backup drive as another partition for storing other data. However partitioning drives can be a bit more complicated these days (especially with APFS volumes), so you might want to have an experienced Mac expert set it up for you. Or you could just bite the bullet and pick up a greater-than18 TB backup drive..