How to create an airline headphone adapter?
The sockets are wired exactly as you'd expect them to be, with the sleeve of each jack as the ground for each channel, and the signal at the tip. The grounds must be commoned within the panel, as I was handed some free headphones on one flight, that had a novel connector to allow them to be used with either the two separate jacks, or a single TRS (tip-ring-sleeve) jack - probably that airline had a fleet with both. 

On that connector, both headphones were grounded to the sleeve of one of the jacks, which was a TRS type, and the other jack was a TS type, with nothing connected to the sleeve, and the jack was hinged to allow it to be folded into the body, at which point a protrusion from the back of the hinged jack, that was presumably part of the turned component that formed the tip, then came into contact with a leaf inside the body that connected it to the ring on the other jack, to produce the standard TRS wiring.

So to make the adapter, you take the ground from the sleeve of the TRS female, and connect it to the sleeve of either of the two male TS jacks, and the ring and tip connections go to the two tips on the male jacks. The sleeve of the other male doesn't need to connect to anything, but could be commoned to the sleeve of the first one.