When we were little, my sister and I would turn one of our bedrooms into an "apartment" by making the top of the bed the "upstairs" and the rest of the floor "downstairs." We even had a secret room in our "attic space" by climbing to the top of the built-in bookcase in her room and snuggly fitting ourselves onto the top shelf, only to pretend it was quite roomy.
Making a room into a loft seems like it's more of an option now, with these cool set ups. Look at the whole collection here. It's inspiring - especially if you're thinking of fitting two big boys into one small room.