Guests using 2nd Bedroom when only booked for two people.

How can I change my set up to reflect if a guest wants to use 2 bedrooms or one bedroom.

I have extra charges for additional guests, but we lot of people booking for two and using all three beds (Two rooms).  As they only book for two there is no additional charge and we have extra room, and linen to change, wash and iron.

Is it possible to set up as Bedroom 1 and 2 ?  If so please send me direction.