As Diana and Don say, list each room separately, then decide what you want to charge for each room if there was one person in each, and set that as your base price, and then set the charge for 'each guest more than 1 person' at a bit less, to encourage them to book two people into one room. If you don't want anyone else to book after you have a booking for one group in one room, I think you'll just have to block the other room manually.
If you have two people booking one room, I would lock the other room, to prevent people like me who would use the other bedroom for their luggage, help themselves to extra pillows and coathangers, just because they think they have rented the whole space. You can tell them in 'Access' that they only have access to one bedroom.
Good luck!
Julia