This will only really work with two sets of Bluetooth headphones that have their own volume controls. If you try it with a wired set, that wired set will always be at full system volume. If you're watching a video or listening to music in iTunes, you can use the iTunes volume slider to set the "max" volume (which will be the volume for the wired headphones) and then adjust from there with the Bluetooth headphones' internal volume controls.Coincidentally, I tried this on a flight recently. I used one pair of bluetooth headphones and one pair of wired headphones. However, when I selected the multi-output device, I couldn't change the volume anymore. It's grayed out and at max volume. Any ideas?