Multi-destination screen streaming

[flightmansam]

What is the feature:
Implement BetterDisplay to be able to stream a virtual display to two destinations (with per-display custom cropping available). For an extra bonus, bundle this into a macro/plugin that can auto-detect how to configure two selected displays to be a "super-display".

Rationale:
I have a use case where at work I have two small 24" 16:9 1080p monitors (yuck!). I rotate them both 90º and butt the thin bezels together to make a 18:16 (9:8) screen. I then have been using BetterDisplay to make a virtual display that is 9:8 and 2160 x 1920 and then stream one half of that screen to on of my physical monitors and then PiP the other half to the other monitor (using screen cropping). This works fine, but would be nicer just to be able to stream both together as the PiP is cumbersome and then is put into a macOS 'space'.

[waydabber]

Somewhat related to this: #200