Created two APFS container partitions on single SSD and now can't resize the one I want

I have a 1TB Samsung 960 PRO and originally set up with a single APFS partition of about 750GB running High Sierra, with a second partition of the remaining free space reserved in case I wanted to later install Windows, Ubuntu etc.

I am also running an nVidia 1080Ti however and did not read the online warnings before upgrading to Mojave. Upshot: I now had a dead video card.

Luckily I already had created the second partition, so I ended up reinstalling High Sierra on that - functional video card again.

The problem is that I have now run out of space on the High Sierra partition and want to expand it. I reduced the size of the Mojave partition to 300GB, leaving me with 470GB of free space, but I am unable to add any of this to the HS partition - at a guess from reading around, because the spaces are not contiguous?

So I am now wondering about my options. I cannot delete the Mojave partition as it is the first on the disk (other than this, I'm not bothered about keeping it). I read also that two APFS partitions on the same physical disk is doing it wrong anyway - you should use volumes, even if you want to install multiple macOS versions?

What I would like to know: is it possible to convert the two macOS partitions to volumes on a single APFS container partition? Or how can I wax the Mojave partition and replace it with the existing HS one?