@Rexhina0 - Always provide the nicer room - the one with the balcony - to your guests first. So this way, the double bed and balcony will always be in use when you have a guest stay regardless if it is 1 person or 5.
My suggestion: Your listing says it is for 2 rooms with 4 beds can sleep up to 5. It looks like you charge an additional $7 US per guest per night. My suggestion is to price the "entire" apartment starting for 2 people a night ($28 US) with a note in your listing and house rules that if 2 guests need BOTH rooms that there is a surcharge of $10 (or whatever you want to charge to make up 2 beds for 2 people) to set up both rooms (and then only set up one of the twin beds in the second room with sheets). When the listing goes to 3 guests, they will pay the extra $7 and get the second room anyway. This way a single person is paying for the larger bed every time.
You WILL have to negotiate this issue every time there are 2 people. So when you get the reservation for 2 you will need to ask - will you require only one bedroom or 2 and then tell them you will send a Resolution Center request for the additional room payment when they check in if they say they need 2 rooms.
You might also consider getting a lock for the twin room and a cute sign that you can post on it for when you only have 1 or 2 guests staying in one larger balcony room. Sign should say something nice like "Please enjoy your stay in the Balcony Room" and then lock the door so they cannot use those beds too.
