Mike, you already have more experience at adding screens than most other forum participants. It would be hard for us to tell you something new. I did twin my theater before I re-opened it, but I don't have any numbers from the days when it was a single screen. I do think a 3rd screen would have been good if I had room for it. Then I would not have been forced to go without getting a new movie on a weekend because of 2, 3, or 4 week commitments tying up my two screens. This happened a few times, and it was not good. I'm sure any twin or single that plays movies on the break will have the same problems.
The things that make a theater owner hesitate before adding screens are lack of space, lack of money for construction and equipment, and the realization that you lose total seating capacity when you divide up existing auditorium space.
As for having twinned my theater, I can say that we lost lots of seating because of the redesign. But, I don't have any regrets since it did make it possible to open more movies on the break. Without the 2nd screen, I would have been in the same tied up boat every time I got a movie on the break. In a lower population area, holding the same movie for 2 or 3 weeks without anything new is bad for business.