Typically I develop my websites on trunk, then merge changes to a testing branch where they are put on a \'beta\' website, and then finally they are merged onto a live branc
The way I and my partner did it was we each made our own private Facebook applications, that pointed to our IP address where we worked on it. Since we worked in the same place, we each picked a different port, and had our router forward that port to our local IP address. It was kinda slow to refresh a page, but it worked very nicely.