@Chris921
Yeah unfortunately Chris this is the loophole in the Airbnb booking system. They wanted to make it as simple as possible and not offer too many booking scenarios to confuse guests.
Without going through the convoluted process of requesting more money from the guest after booking, you can charge by the guest, but not by the bed.
I have the same situation Chris, I charge $85 per night for one or two people. I charge $100 pr night when a third person is nominated because that means they will use the second bed. But on a few occasions I get two friends travelling together, they see that I offer a second bed in my description and require the $85 per night rate simply because there is only two of them....That is my two person rate. I can't charge by the bed, I can only charge by the guest.
In these instances Chris I just suck it up and take the hit where the extra servicing is concerned. Some guests will be good enough to say, can I give you extra to make up that second bed, but I don't accept it, I just tell them it's ok and the extra work will do me good! Guarantees me of a good review, and Chris this is where it becomes money in the bank....in those good reviews.
Sometimes it is worth forgoing a dollar in order to make a dollar Chris. It is a failing in the system, sure, but you sure as hell are not going to get Airbnb to do anything about it. The phylosophy is, better the host take the hit than Airbnb!
Cheers......Rob