If I understand your question correctly, you have a shared room space with 2 individual beds for guests and you're trying to give people the option of reserving a single bed (and another person reserve the other) or they can reserve 2 beds.
You can do this a few different ways:
1) You can make 2 listings - one for each bed - with max of one person, and tell guests that they can reserve both listings if they are traveling with 2 people. You may want to specify which bed (top or bottom bunk) is reserved for each listing. This requires the guest to do extra work with 2 reservations, and also means you don't come up in listings when people search for rooms for 2 people, so possibly not your best option.
2) You can make 3 listings - 1 listing for Bed A (max 1-person), 1 listing for Bed B (max 1-person), and a 3rd listing that's for both beds (max 2-person). In your availability settings, be sure to synch the calendars (so when Bed A is booked, the 2-bed listing is blocked, or when the 2-bed listing is blocked, the individual bed listings can't be booked).
Airbnb has a help article here, you're listing beds rather than rooms, but same advice applies:
https://www.airbnb.com/help/article/289/how-do-i-list-multiple-rooms