Do you charge extra for more than 2 people?
If you do, then i would lock the second bedroom because it encourages folks to bring a friend or two along - hey, there’s a spare bed, join us!
You could also ask the guests. Probably not “are you going to share a bed” but “Were you planning on using the second bedroom?”
Though if i were a guest renting a whole house (if that’s what you have?) in which the listing said “2 bedrooms” i would assume i had access to both, no matter the size of my party.
Does that help?