When I taught close formation flying in the VC10, it was common to teach both the captain and co-pilot on the same flight. So a double seat swap for the instructor was quite normal - but there was always a pilot in one seat or other, fully strapped in and briefed.
Taking over a seat from someone who'd been sweating away flying close formation for half an hour or so was never fun!
We also did double seat swaps in the holding pattern whilst training, but always with one pilot strapped in and the autopilot engaged.
But NEVER a seat swap with no-one in a pilot's seat - that's madness!
One Air Eng held a pilot licence and once asked his captain for a landing. "Certainly", said the captain, "but first you must apply for a commission as a pilot, then if you pass the course, complete advanced flying training and get posted to the VC10, THEN you can have a landing!"