anchored split screen across desktops
I've come up with one approach for the LG 34" 5k2k screen...

On the same mac, I connected with a thunderbolt 3 cable, and then via an adaptor I connected the same mac via HDMI, the solution being the mac thinks you're connecting to two screens, and the LG monitor thinks it's receiving two input signals which can then be displayed via PBP mode.

I have a 75/25 split and I'd say the resolution on the 25 side needs to be modified so text is readable which for me is at 800x1350, which doesn't give much space but just about works... I wish it supported 70/30 and it'd probably be better.

If someone has a single-cable split screen solution for what I describe, I'd love to hear it!