Dual plane breast augmentation places the breast implants underneath the muscle. Typically a small "lateral sling" of muscle was left intact to hold the implant in place and prevent sagging. In my opinion there is much less chance that sub-muscular "dual plane" breast implants will sag than when implants are placed on top of the muscle.
The answer this question is a little bit complicated. What the dual plane really means is that, where is the transition of the pocket from under the muscle to just under the lining of the bottom muscles. If the bottom of the pectoralis muscle is still intact below the midportion of the implant, then in theory the implant will never settle. The overlying breast, which is not attached to the implant still can fall leaving the implant above causing what we call a Snoopy's nose deformity. When the breast has settled a little bit on the chest wall, such as after childbirth or weight loss, then the nipple is no longer above the bottom portion of the pectoralis muscle. It is then necessary to divide some of this muscle fiber to allow the middle of the implant to line up with the nipple. As more and more muscle is released, especially as we go to the level of the top of the areola, there is then essentially no muscle to hold the implant up and this implant will descend with the breast quite similar to what a sub glandular implant would do. I hope this answers your question.