Back Up Drive format

If you are on macOS Catalina your boot disk has to be APFS and so must any bootable clone.

Ditto for Mojave except I have seen some complex (and risky) workarounds to install Mojave APFS and then do a conversion to HFS+. Myself, I wouldn't do it.

With High Sierra you can use either HFS+ or APFS but getting it on HFS+ can sometimes get tricky especially if you are using SSDs and/or doing a fresh install.

As an alternative to cloning...

Here’s the official sequence for creating a bootable installer:

How to create a bootable installer for macOS - Apple Support

That’ll then be able to install, and to also then restore a Time Machine backup.

A Time Machine backup itself can also be booted.

For your purposes I would do 2 partitions, as you indicated. They can both be APFS if you want them to be, it will work fine.

If it were me, however, I would make the second partition HFS+ (macOS Extended Journaled) since you are using a traditional hard drive and the second partition will only contain data.

On the backup drive you can just partition it 500/500 GB. and the format can be APFS or not. Standard backup software (like CCC or ChronoSync) will back up to either Apple format. When you go to create a clone, both CCC and SuperDuper! will take care of the formatting and partitioning when creating a clone. With CCC you'll end up with one "Macintosh HD Clone", with SuperDuper! you'll end up with a "Macintosh HD Clone" and a "Macintosh HD - Data Clone." Just a slightly different way each developer does it but they both end up providing a bullet proof safety net. Can't go wrong with either.