I have a pair of Sennheiser PXC-450s and they come with a 2-1 jack adapter. The only problem is that the input to the adapter is angled at 90 degrees and in J the jack has a surround on it. I haven't had a problem with the 744 seats but on the 77W I've had trouble getting the adapter to clear the surround.

You don't need to put anything in the third jack as this is used for the noise cancelling feature which is built into the IFE rather than the headset itself.

If you are looking for a splitter/adapter then I suggest Radio Shack. Doh. No Radio Shack in HKG but finding the item shouldn't be hard there!