There's a manga called Love/Death that is about two yandere girls being into each other. Rather than cancelling out and only leaving the dere, it's more like yandere squared.
when two yandere get together you just worship each other until it becomes like a feedback loop in a speaker system drowning out the rest of the world and deafening both of you and you lose track of what's normal. it's a lovely greedy fire-thing that eats you up piece by piece and makes you want to feed it with all the rest of your life. she drank some of my blood because it was romantic and then vomited it up but her lips were sugar-sticky