Karla,
I have a 557 seat single screen with full stage facilities, and that is hardly enough to make live presentations work. If you wish to bring in touring acts, it is difficult to make the numbers work with that little seating, but if you're going to do local programing, theatre, concerts etc. it's a nice comfortable number.
As SamCat suggested, you will need full support facilities to do stage presentations, a 15' deep by 30' wide staging area as a minimum (more is much better),an overhead flyloft for flying the movie screen and hanging curtains, backdrops and scenery. Wing space on each side of the stage. An orchestra pit for about 10 musicians. Several dressing rooms & rest rooms, 3 at least, 4 or 5 is much better. A green room (performers lounge) is nice. An acessible load in area to the rear of the stage. If you're going to be doing plays with a stock company a scene shop and scenery dock for building and storing scenery would be a major plus.
A 2nd smaller auditorium to keep the movie patrons coming during live shows would be nice as well, but I wouldn't give up support space for the live theatre just to get that 2nd screen, otherwise the live theatre will never work well, so why even do it. If you're going to do live theatre, do it right, or don't do it at all.