Imo, don't think this is a good idea, hair touching could also be confused with "touching the hair (with the hands/other)", but we already have the tags:
hand_in_own_hair (one's own)
hand_in_another's_hair (other people's)
and a number of other tags for more specific actions.
Whereas for the action you mean, I see that the hair-to-hair tag has 20 posts (of which many are parents) and that, essentially, describes what is already described by the heads_together tag (and sometimes both are tagged).
I would be of the opinion to nuke hair-to-hair or create an alias:
hair-to-hair -> heads_together
although the number of posts is so low that it can be done effortlessly by hand.
If it is the actual distance of the heads that creates a problem (hair touch but without real contact between the skin of the heads, for example) then I can understand, but in this case we would need to clean up the entire heads_together tag, which has numerous examples of this type.
Moreover, there are even tags that specify the parts that are being touched.