Airbnb's setup is to charge by the person, not the bed, so I think it's best to work within that system.
Some groups use amenities/consumables much more than others, whether that's coffee, TP, towels, or extra blankets. Other groups use none of those things. Using extra beds is kind of the same.
If it really bugs you, I'd set up additional listings so you can sell a single-room to smaller groups and lock off the second. We considered that, but found it too fussy. I charge and prep for a max of 4 guests in 3 beds, with the expectation they'll all be used. It's a happy surprise if they aren't.
The only thing I'd mention to guests is that both beds are identical (if you think the extra use is due to them trying out beds like goldilocks). I wouldn't mention anything about what beds I expected them to sleep in, or that I hoped they'd only use one.