Sharing GUI windows between Macs
Well, if you start an Application that uses X11, just like that : ssh -X user@host

You can run most "Linux GUI" X11 applications on mac as described here using docker: How to run a Linux GUI application on OSX using Docker

Make sure to check out XPRA for Mac, too.
You can forward X11 apps on mac or forward the whole mac desktop using xpra, to install do

brew install Caskroom/cask/xpra


to shadow the desktop

/Applications/Xpra.app/Contents/Xpra_NoDock.app/Contents/MacOS/Xpra  shadow --bind-tcp=127.0.0.1:10000


etc.pp.