It looks like the terms have been aliased. Would it be better to have an implication of symmetrical_docking to breast_to_breast and remove the symmetrical_docking tag in cases where breasts are touching but not 'interlocking'?
e.g.:
http://miezaru.donmai.us/post/show/130720
http://miezaru.donmai.us/post/show/127990
http://miezaru.donmai.us/post/show/130228 (borderline)