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Oh fuck, that sounds like a minami haruka manga lol.
The manga I'm talking about. It might not be what you're looking for but the resemblance to your description is almost uncanny. http://www.mangago.zone/read-manga/kohitsuji_hokaku_keikaku/
omg thank you! yes this is it HAHHAHAHA THANKS A BUNCH
2 twins uke who would sleep together,
one of them was a seme but turns to uke later by a seme they met in school. the 2 seme had actually been secretly watching the 2 twins sleeping together.
thank you!!