I'm flying this Friday again and can't find my two-to-one 3.5 mm jack adapter and I was wondering why did they decided to go this way?
One like this:
I know why they were using the two prongs, to avoid of theft of the airline distributed headphones. But why not use two regular jacks and just switch the channels on the second one? This way everybody could use his own headphones.
I'll speculate on your main question: Not only did the non-standard jacks mean that the airline's equipment would be useless outside of the airplane, it also meant you had to use their equipment if you wanted to listen to audio. Meaning, the airline could also charge you for it.
I can only assume it was to prevent people from stealing their headphones. In response to your second question, I have often used my own headphones on planes that have the two jacks, with mixed results.
Most of the time you only get one audio channel when you do (as you might expect), which means you only hear out of one ear. This typically does not bother me.
Other times, if you pull out the jack slightly, you can get sound out of both ears. Its nice when this happens, although its sometimes a little tricky to keep the jack out slightly (it tends to fall out).
The reason was compatibility with piped sound sockets in older planes.
During the transition from piped to wired sound, the spacing between left and right channels was maintained as it suited the metal template in the individual passengers consoles
Well, as for "why", others have answered nicely. If you wanted to get around it without using their equipment, you could get a Y-cable in configuration 3.5 TRS F -- 2x3.5 TS M. I tried looking for it online, it seems to be uncommon, so you can either use two cables/adaptors that are available in the music stores (for instance 3.5 TRS M -- 2x3.5 TS M and 3.5 TRS F -- 3.5 TRS F), or solder one yourself.
Explanation to the vocabulary:
Your headphones have a stereo male jack, which is 3.5 TRS M. It fits into a female 3.5 TRS F. The mount in the place is two mono female outlets, which is 2x3.5 TS F, and 2x3.5 TS M fits in them. So, you need a cable with 3.5 TRS F -- 2x3.5 TS M, or a combination of cables that give you this configuration. This is exactly what you show in the picture, however, there the two mono jacks are in a fixed distance, if you use cables, you get variable distance, hence less troubles.
On some aircraft the second jack is used to provide electrical power for noise cancelling headphones. On most of the aircraft I've been on recently, the 3.5 mm jack is the norm. The two pronged configuration, with one prong for each stereo channel, is becoming uncommon in Europe and North/South America.