HSF+ and APFS
 

I used to have two backup disks, one used for Time Machine, and one used for SuperDuper clones. A while ago, I started putting two partitions on each physical drive, one partition for Time Machine and one for SuperDuper. All these drives were HFS+ and used with older Macs with older OS. This meant I had one of each type of backup on each of two external drives.

Now, with a new Mac and Catalina, I have separate physical disks used for Time Machine (HFS+) and for SuperDuper (APFS). Of course the original internal drive being backed up is APFS. But this means I have only a single instance of each type of backup, whereas before I had two of each (on two physical drives).

Is it possible to place an HFS+ partition and an APFS partition both on an external drive and do what I was doing before?

Yes, you can do that, but the drives may not be bootable. You can try it...