Hi Chloe
I know exactly what you mean
My costs are relational to the number of beds that have been used, and not necessarily the number of guests.
My place has 3 bedrooms, sleeps up to 7 people in total, and has 5 individual beds.(2 doubles and 3 singles)
My base rate is for 4 people, and £10 p/p p/n for each 5th, 6th or 7th person.
I often receive bookings for 4 people, but they use a bed each. So i have twice as much washing to do, but don't get anything extra for it. So far, I just accept it, and console myself that there isn't as many towels to wash.
Pehaps you could state in your House Rules, that for a 2 person booking, only one bedroom will be made available. If both bedrooms are required, then please let me know. You could then offer the second bedroom, but explain that a sur-charge will have to be made