COY stands for Cross Over Yuri. The premise is pretty simple, I have a list I've compiled of over 500 cute girls from various anime and manga. I then use a random number generator to make pairing, which I then draw. Every third picture is a threesome.
This was the second COY pic I did. Not too fond of this one, something about it just feels very off to me.
Again, though, this was an ideal pairing for something randomly generated. Mutsumi and Hyatt are both very similar characters, and may even share a voice actress (if not in Japanese, then in English).