@Stephanie268: the only real way to do that is to write it in your listing that this listing is only for one room. You could say something like, "If you want to book the second room the cost is $XX additional. Please send me an inquiry and I will return your message with an offer that includes the full price."
You should be very clear in the listing as well as your pictures that the second room is not part of the initial price to make sure there is no confusion.
Another thought, if you have a second room available to rent, why not create a second listing and rent both of them as one-room listings and earn more money? In that manner you could say that you have another room available that can be booked if not already reserved and please visit this URL to view it.