For what its worth, many here frequently referenced the "Master's of the Barebow" DVD series, with Vol III seeming to be the most helpful as a 'coaching' device. Therefore you might be aware that Larry Yien uses the term "double anchor" also. However, he explains it differently.
The first anchor is the drawing hand to face anchor, and the second is the locking into the back muscles anchor. But, Larry explains that locking into the back muscles 'anchor' does not move the draw hand anchor at the face, because locking into the back muscles is not done by a continuing rearward motion of the drawing arm, but by a 'rotation' of the elbow clockwise (RH shooter) toward the spine and locking into those muscles. So, full draw is attained when the anchor point is reached on the face with the proper arrow tip to elbow alignment, and there is no further 'rearward' arm movement.
He further explains that there should be little to no tension in the drawing arm/shoulder, other than what is needed to maintain the "hook" on the string. In other words, the arm & shoulder become a relaxed link between the set 'hook' and the back muscles.
It's another case of use whatever continues to work best for you. There is no 'right' or 'wrong'. IMHO, Consistency is what it's all about. Larry's method has certainly done well by him as the 3 time IFAA champion.