Kyouya/Hikaru. Kyouya as seme, already established Kyouya/Kaoru relationship. Hikaru is jealous and poses as Kaoru, either because he wants Kyouya himself or he doesn't want Kyouya touching Kaoru. Points for Hikaru actually getting turned on by being called Kaoru.
Do hope that isn't too confusing!
Oh, yeah! Whether or not Kyouya figures out he's with the wrong guy is up to whoever writes it. ;)