I think noise cancelling is only practical at the listening location.

Cancel at the source tech definitely exists, but it generally works best for repetitive predictable noise (same with noise cancelling in general)

Here's an example: https://www.silentium.com/technology-2/quiet-at-the-source/

Yep. Many years ago I was a junior at an engineering consultancy that specialised in noise & vibration. I spent many a happy hour out in the sun replacing melted loudspeakers that were being used to feed 'anti noise' into the exhaust ports of a pair of RB211 jet engines. The engines were ground mounted vertically to introduce pressure into the gas network should an emergency loss arise. This was rural Cheshire (UK) and some of the local properties were very old and expensive and needed protection from the vibrations these things produced. In reality they were never used but they were tested twice a year, for a total of about 2 minutes. After that the speakers (yep lots of hi-fi cones) were mush. They did their job though. And I got to travel up to Cheshire, see my then girlfriend who lived in Manchester and spend a week up on a roof in the sunshine replacing them.

I wonder what the result would be of using pulsejets offset in timing by 50% of eachother.

Lots of weird mechanical resonance, and not noise cancellation.

Turbofan noise is nothing compared to sonic boom noise.