It does happen- you have to be accepted by both faculties seperately so if one accepts you and the other doesn't, you may well get an offer for single honours.
I do single honours Classics and judging from my friend who does joint with English, it sounds like a tough (she has to do her mods next term, whereas I don't do mine until this time next year) but incredibly interesting course and they really make the effort to link between the two rather than you just having two halves of a degree that don't really fit together. Would you be course I or II for Classics?