@Erin205 you are not the only person with the same issue.
A "per-guest" charge only sort-of works... sometimes 2 guests take up 1 room, sometimes 2 guests take up 2 rooms.
I have yet to read of a solution that does not involve a fair amount of manual work on the part of the host: messaging, explaining, adjust charges, and such.
The closest would be to set up a per-guest charge that starts after 2 guests. 1 or 2 guests is a certain amount, adding a 3rd guest increases the price.
Perhaps you can explain in the description that 2 guests only get one room, and if they want 2 rooms they should put in 3 guests (but not 4).
(BTW, your place looks quite nice.)
Someday in the future, you might consider making the rooms separate listing, though. You are right: you have a hot commodity.