I'm a fairly new host, but I think I would handle it this way:
If this only happens now and again, let it slide. I have a similar issue in that I list the master bedroom with attached bathroom. The bathroom has a stand-up shower in it, and I say in my house rules that guests are welcome to use the main bathroom in the house if they want to use the bathtub in there. If guests use the second bathroom, it's extra cleaning for me... but I don't charge extra for the occasional guest who chooses to do that. It wouldn't be worth the hassle.
If this is a fairly frequent occurance, you could have a charge for the additional guest. That is, set your room rate for one person, with an additonal charge for the second guest. Sometimes that second guest will share the bed, sometimes they'll use your couch. You'll get paid either way, and your guests won't think anything of it.
I feel it's kind of tacky to ask for additional payment for things, but the Airbnb website is set up to charge for additional guests, so I'd use that feature here.
Jamie