See Packeagles' post (#12) in this thread
It's the solution I'm using to my hanging problem - a very small bedroom in a century old house with questionable framing of newer inside walls. To add to the problem, it's a rental where I was really, really loath to make a mess of the walls or the ceiling since I have one of the bestest landlords ever!!
I'll be making the frame from 4 x 4 uprights and 2x4 for the cross beams. It could easily be adapted for whatever size of room you have and with extra supports either work for a double hammock or two singles.