The technique to rolling a double sea kayak is no different than rolling a single kayak, with one exception. Syncopation. You have to roll together or all bets are off. The other technique is that either the front paddler tucks forward and the rear paddler rolls both up or vice-versa.
Obviously Shawna and Kathy have the technique down.
It is also really good fun. Doug Van Doren and I did this in the pool with a homemade double a while back. That Nigel Dennis Triton II Sea Kayak is the fastest double I have paddled yet. I really like it.
As we all know the greenlandic rolling jacket the tuiliq, makes rolling a lot easier. The flotation in the jacket is almost cheating. Both women appear to be wearing them.