Edit
When two girls press their breasts (of roughly equal size) against each other. If there is a noticeable size difference, it is asymmetrical docking instead.
The term originates from Yuusha ou GaoGaiGar, in which the combined form of Hyouryuu and Enryuu, Chouryujin, was released through a maneuver called "symmetrical docking" (Danbooru post #132179). Later, other similar paired mecha performed the same combining maneuver.
In a Super Robot Wars 4koma (Danbooru post #150779), the maneuver was likened to the action the tag now stands for.
If the breast(s) are resting on top of each other instead of being pushed against each other please use breast on breast instead.
Examples
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