Double boa tightens the lower boot and upper boot separately, same as Burton speed lace.
Not all of them. The K2 Contour has double boa, but one is for the outer boot, one for the INNER = you can adjust the inner boot easily. If you tightem them "normally" they are mid-stiff, if tightened hard they are rather stiff, what I like.
The boot has two main advantages for me. 1st, the open very wide. I have a high arch and entering the Burtons I had before whas a tiring undertaking of several minutes pushing squeezing hopping and swearing. The Contours open wide enough to put them on like slippers. 2 seconds.
2nd: I prefer to say that I got complicated feet but guess, that I
. I never felt comfortable with boots where you have to deside in the morning how tight you bind your inner boot... I have different demands over a day riding! The day starts with a hike: no need to have the boots very tight. For the runs on groomers I want them as tight as the material allows. In pow I prefer them mid stiff. Loose on the chairlift and lunch break. No problem with the Contour. They are the first boots that allow me to ride an entire day with hard tightened boots without getting foot pain, numb toes, getting cold feet. At the top of a run I tighten them (and the bindings) hard, after two turns I stop for some seconds to squeeze another notches and boa turns. And then you can loosen the outer AND inner boot at the chairlift on the fly to allow full blood circulation.
They give me the freedon to find the perfect amount of thightness for every instance.
They also were very comfortable with hiking cause of this on the fly adjustability. You don't even have to stop walking to tighten or release them bit by bit to find the most comfortable fit. If I knew that this separate boa for the inner boot would be discontinued, I'd buy 10 pairs of them to have enough of them in stock for the rest of my riding life
But that's me. Tried to point out the main advantages I
think are important