What is the correct way to format a 1TB internal SSD, into 2 equal APFS volumes?

Within Disk Utility click "View" and select "Show All Devices" so the physical drives appear on the left pane of Disk Utility. Erase the whole physical drive as GUID partition and APFS. Once you have the single partition/volume on the drive, then select the Partition tab and resize the existing partition until you have the sizes you want for each. Click on the second new partition and make sure to select APFS for the filesystem and give it a name.

Depending on what your goal is here, you may want to instead consider adding a second APFS volume instead of a partition. The second APFS volume will reside in the same APFS container, but will still be separate from the first APFS volume much like a partition. Each APFS volume can be individually mounted & unmounted. All APFS volumes residing within a single Container do share the same storage pool, but doesn't require any hard size restrictions like a separate partition would impose. Most people who use multiple partitions will usually discover that one or more of those partitions are too small requiring the user to start all over again.

Correct.

I've never tried to install two operating systems into a single APFS container before, but I believe I have seen a couple of respected forum contributors mention it is Ok. For High Sierra & Mojave, I think it will be fine, you will need to give them unique volume names. Things get a bit tricker with macOS 10.15+ since later versions of macOS utilize multiple APFS volumes anyway.

If installing High Sierra & Mojave into the same APFS container (separate APFS volumes for each within that Container), then if you decide to get rid of one those operating system, you should just be able to delete that APFS volume. Your remaining OS should have the full storage space for itself without having to perform a risky resizing of partitions.

Actually I just had the opportunity to install a second copy of macOS into another APFS volume within the same Container as the other OS. Everything went well. Probably the only time you may have a bit of an issue is when an older version of macOS 10.13 or 10.14 is installed alongside macOS 10.15+ since those older versions of macOS may complain about the "Update" volume utilized by macOS 10.15+.

The trick is to give the new APFS volume a unique name to distinguish it from your main/first boot volume.