@Gaye45 Some thoughts
Just because only 2 book you cannot stop them using both bedrooms. The best solution for you is to create two listings
A 1 bed listing that takes 2 people for price X
A 2 bed listing that takes 4 people for price X++
Then link the listings to avoid double bookings.
This way you can then legitimately lock the 2nd bedroom when the 1 bed listing is booked.