A double anchor is simply utilizing two distinct points to maximize consistency. Nothing new but not everyone uses this approach. Rick Welch teaches the thumb knuckle touching the ear lobe and the cock feather on the tip of the nose. Byron Ferguson uses middle finger on a specific tooth and his index/ ring finger on points of his face. Ty Pelfry (stringwalker) uses the thumb under the jawbone, index against the cheekbone, and arrow nock against the nose.
Normally, the double anchor is grounded on a bony part of the face (cheek bone, back of jaw, tooth, etc) to give a solid landmark. The common corner of the mouth is more nebulous. It can move if you open your jaw, if you're cold, purse the lips, etc.
Different archers use different points. Your facial structure dictate what may work. All it really does is help you find a consistent point to align your head, hand, and body position.
Dozer, if you want to try a double anchor you might try holding at full draw for a moment to make sure the shot is solid. At full draw feel the anchor, hold it, solidify the sight picture, then release. Once you learn a double anchor you can shoot as quickly or slowly as you want. It will just help you be more consistent from shot to shot.