Nipple to nipple is fantastic, but as noted above, it's pretty rare.
I always wondered if women who fight/wrestle topless can feel their nipples press into their opponent's nipples, or if their nipples slip past their opponent's nipples as their tits slide past one another. If their tits slide side to side against each other, do their nipples "catch" on their opponent's nipples? If both women's nipples are erect because of close tit to tit contact, can they feel their hard nipples pressing into their opponent's?
Would love to hear from the ladies on this subject, if they have had this experience.