Create 2 resizable logical drives from one physical drive under Catalina
Here is Apple's guide on the much better way of using containers to hold all the space on a drive and then make resizable and shares space logical volumes.

https://support.apple.com/guide/disk-utility/add-erase-or-delete-apfs-volumes-dskua9e6a110/mac

In practice this is so much better than the old way of having to guess beforehand how much space each needed and being forced to jump through hops to grow and shrink volumes. APFS does need SSD to run fast, but you gain so much flexibility to create new volumes it's worth selling an old Mac to get a new one IMO. I don't recommend this on Fusion Drives - there, try to void any split unless you can't get external SSD for a better APFS experience.
If you like hearing how this works, watch this video alerting at 12 minutes in on the benefits of space sharing...

https://developer.apple.com/wwdc17/715 Check out What's New in Apple File System

As to how to add, back to the first link. I have never needed to reserve space or use quotas, but feel free to experiment if you feel it’s better to restrict things for your setup.