I was recently watching the latest season of Star Trek: Discovery. While watching the opening credits, I noticed that two episodes of season 4 had two directors listed for those two episodes: Christopher J Bryne & Jen McGowan (4.08), and Jeff Byrd & (again) Jen Mcgowan (4.11).
Now I have seen pairs of screen writers (such as Alexander Kurtzman and former writing partner Roberto Orci) and I know that there are assistant directors, special scene choreographers, and/or cinematographers/art of Photography that can be additional to any given director, but I don't recall seeing a double director feature film or TV series episode before.
What was the first film project to do this, and what is the general/common purpose if it?