OS kernel development on one, with the second computer connected to the serial port (or moral equivalent) for diagnostics.
One machine is dedicated to benchmarking, while the other is where you have a web browser with 14 open windows and 132 tabs, and a video call going, all causing jitter.
I've had a job where my client sent me a computer to use for their projects, and only their projects.
Configuration like you mention is a different matter.
If you don't mind me asking, what type of work requires an airgap?
A military friend of mine deployed in Iraq in 2009 could contact me on one computer system, but the other was was locked tight, eg, with glue filling the USB slots.
Bruce Schneier writes at https://www.theguardian.com/world/2013/sep/05/nsa-how-to-rem... :
> Since I started working with the Snowden documents, I bought a new computer that has never been connected to the internet. If I want to transfer a file, I encrypt the file on the secure computer and walk it over to my internet computer, using a USB stick. To decrypt something, I reverse the process. This might not be bulletproof, but it's pretty good.
At https://www.wired.com/2013/10/149481/ he also writes "Osama Bin Laden used one. I hope human rights organizations in repressive countries are doing the same." and "the Natanz nuclear plant in Iran" used one as well.