It's worth noting that the newer Bose bluetooth headphones with the newest firmware can do this with any bluetooth source (including iOS devices). You can use the Bose app to temporarily link two of their bluetooth headphones together. For example, the audio goes from your iPad to the first set of headphones, and then those headphones transmit it to the second set of headphones. There is no lag between the headphones, and the audio stays in sync with the video you're watching.
I just did this a few weeks ago with my wife, and it was surprisingly great. The only downside was that the headphone batteries seemed to drain a little faster than normal, but I can't be sure about that.