murphy and paddy

Murphy calls to see his mate Paddy who has a broken leg.
Paddy says, "Me feet are freezing mate, could you nip upstairs and get me slippers?"
"No bother," he says, and he runs upstairs and there are Paddy's two stunning 19 year old twin daughters
sat on their beds.
"Hello dere girls, your Da' sent me up here to shag ya both."
"Fook off you liar!".
"I'll prove it," Murphy says.
So he shouts down the stairs, "Both of them, Paddy?"
"Of course, what's the use of fookin' one?"