One SSD partitioned into APFS and ExFAT?

You should be able to do that. First you would partition and format the drive into two equal partitions as OS X Extended (journaled) or whatever size ratio you need. Then select one partition and reformat as APFS and then the other partition as ExFAT.

The reason for starting out as OS X Extended is that once formatted as APFS it's very difficult to get to ExFAT or OS X Extended. So start with OSXE and for from there.

And be sure you don't use the APFS for Time Machine like P. Phillips warned.

as cheap as external drives have become, if it was me, I would use independant drives. one for each file system.

Yes possible but perhaps if the User offers what they are attempted to achieve by doing this, we may better understand the why ?

If Time Machine Backup in involved in this setup I would stay away from splitting the drive at all

All eggs are in one Basket and the basket fails and they do, Scrambled Eggs is what we get

The suggest following the advise from the Well Wisher for the New Year @Old Toad 👍