Maximise to half screen and choose other window I might have imagined it but I think I just did something to maximise a window to half fullscreen on a new desktop and then was presented windows to show on the other half, very much like windows does. I'm not sure howe I did it but would like to know how.

I do have Better touch tool which has sort of that functionality but it's not the full screen desktop version - it does the equivalent of Option clicking the green button.

Anybody have any ideas how to actually do this? (I'm on High Sierra, believe it or not).
I worked it out. 

A long press on the green button then allows you to drag it to the half of the screen you want. (Better touch tool not required, default MacOS behaviour)