That's brilliant! exactly what I was looking for.

in terms of minimising lag are there any best practices that I should be aware of?

obviously even with the output and input being the same machine as they are in your example the 2 versions of the video won't be perfectly in sync (although here it doesn't seem to be any more than a couple of frames so probably no-one would ever notice) but it would be good to have a heads up of the kinds of things that might cause problems for when I need to troubleshoot in the future.

Thanks

It's actually quite simple.

Crash Course:

Incoming NDI --> download and use the NDI Watcher plugin from the TroikaTronix Add-Ons page (there are usage instructions for it in the description).

Outgoing NDI --> Just go to Output>Stage Setup and select the Stage you want to output, then check the "NDI" box in the "External Outputs" section of the Stage Settings (bottom center)

Here's an Example File

Here's an example gif of the example file