@Jeanne246 You can also write it on the the listing - telling the guest that even though your listing is not really suitable for more than one it is possible for a set up for two. You can write that they must write to you if they want to book for more than 1 and you can then ask for the extra 20 $ via the resolution center. You can ask for extra money as soon as a reservation is confirmed.
The problem with that solution would be that if a guest search for 2 persons your listing won't show in the results. So maybe just make sure the guest is fully aware of the setup when they book?