You gotta trust us on this one
You'll get the maximum sound transmission loss with two masses separated by an airspace. If the two doors are connected by the frame, they're CONNECTED. They act as one.
If you don't fully understand the physics of sound transmission (I don't) and aren't going to hire an acoustical engineer, at least buy a book and trust those who have done this before. You're getting free advice that an acoustical engineer would charge you hundreds of dollars an hour for.
Buy Charles Salter's book, and look up any of the studies the Joiner-Rose Group has done.
