Sorry @Stacey139, If you did what @Jessica-and-Henry0 or @Sandra856 suggested and I was your guest, I'd be complaining to Airbnb. If you advertise a 2 bedroom, 3 bed space, then that is what you must provide because that's what I've signed up (and paid) for. And 'bed' means a made up bed. Husband and I quite often choose a 2-bed place because we prefer the option of sleeping elsewhere if the bed doesn't suit one of us (he likes much warmer bedding than me), or one of us is snoring. We also like to leave an open suitcase on a bed rather than unpack for a short visit (or any visit if there's nowhere to unpack anything into!).
What you could do is offer a discount if the guests agree to use only one room, or only certain beds.
Alternatively, advertise the rooms separately - one advert for both rooms (higher price), 1 advert for just the double - I believe you can link them somehow so both can't be booked at the same time.