@Peter1609
you can charge them an additional 30$ through the resolution center ( add and receive money) and they should accept it for you to be paid.
Or You can do what @Lisa723 suggested
Or you can do what we are doing - we have the same situation in our apartment - queen bed and twin bed. Sometimes 2 guests book but they want to sleep on 2 separate beds. We don't charge it extra because we have single size duvets so if they sleep together or separate they will get the same 2 duvets and 2 pillows. The only difference is 1 extra sheet.