As long as you allow plenty of circulation around the birds the cooking time should be the same as for one bird. Get an instant-read thermometer and stick it in the thickest part when you think it is done. The thermometer should read at least 165 degrees. Any more only dries it out.
My wife has a tendency to overcook meat which I am trying to get her to unlearn. She doesn't quite understand the concept that you take a steak out of an oven when it is just slightly underdone because it still continues to cook internally. Also the idea that there should be a bit of red or pink in the meat is revolting to her. As a result, when we cook steaks, there are two widely varying times for removing the meat. Typically her's stays in the oven about 20 minutes longer than mine.