hi,
I have never done long haul flights only 3-4 hour ones but what I find useful is to keep to baby's routine as much as possible, ie naps/sleeps etc, my friends who do long haul always recommend taking a night flight (assuming your babies sleep most of the night!) so you can give them their last feed when the plane takes off (to ease ear pressure as mentioned above) and put them down to sleep when you get on board - normally the whole excitment of travel exhausts them enough and the gentle hum is actually quite soothing for them. Remember to pre-book your seats beforehand, you want as much leg room as possible (bulk head seats?) and if possible to be as far away from the toilets as the traffic and noise of flushing toilets will be a little disruptive. If you are not breastfeeding you may have to cart water and formula as I wouldn't like to rely on the water they provide onboard. When you arrive in HK, arrange for a car pick up service (eg. a large MPV that will sit you/hubby/2 babies and numerous luggage!) comfortably, you want your arrival to be as seamless as possible with 2 babies. We use Richwheel limo services - HK$500 from airport to our apt (HK side), speak to Samson at +852 90311575.
Have wonderful trip to HK!