My experience is that a lot of landlords aren't too happy with this sort of thing, I think it just adds an extra difficulty (e.g. a different contract, another person to default on rent, what happens if you break up, another potential flatmate conflict). You should just talk to some (and your friends) before you decide on anything, I'm sure you can find something if your friends are happy to compromise a bit. You should be very careful about not upsetting your flatmates though, living with a couple can be awkward.