Video is too large to attach. However....tackler and ball carrier BOTH lower their heads and SIMULTANEOUSLY make helmet to helmet contact with top of the helmets. Is it a foul? And what is the difference in NFHS and NCAA in interpretation of "helmet to helmet"?
Since you specifically ask about NCAA, there is no strict prohibition against contact made "helmet-to-helmet." Players often make contact with an opponent's helmet with their own helmets, i.e., facemask to facemask, side of helmet to opponent's helmet, etc. Such contact is being made constantly by opponents of both teams. Helmet-to-helmet contact is not, inherently, a foul.
The two fouls of note in NCAA are the two types of targeting, one of which - 9-1-3 - requires that a player 'attack' an opponent with 'forcible contact' with the crown of his own helmet, into any - ANY - part of the opponent's person, and that there must be an 'indicator' (a launch; upward thrust; lowering of the head; leading with the head). This is what we always used to call "spearing." And this rule applies to any opponent - not just the ball carrier (BC).
The other is 9-1-4, in which a player makes forcible contact to the head/neck area of a 'defenseless' opponent, with an indicator (as previously described). There is a long list of what makes a player 'defenseless.' A BC is NOT defenseless. So, just because a tackler makes hard contact with his own helmet, to the helmet of the BC (even the star QB), if that contact is NOT with the crown of the helmet, the contact is legal. However, a player attempting to catch a pass or a punt, for example, is 'defenseless,' and that same forcible contact to the head-neck area of that receiver, with an indicator, would be targeting. Note that 9-1-4 targeting does not require that contact by the attacking player be with the head/helmet. It could be with any part of the attacker's body - hand(s), arms(s), shoulder, knee(s), leg(s), foot/feet.
Yes, we roll our eyes every time we hear coaches/fans screaming about "helmet-to-helmet." Not necessarily a problem, Coach. When it IS a problem, Coach, we'll get it.