I usually go to movies and sit between two people. I take claim to both armrests because the other two can suck it up and use the one that's not used by me. I need a place to put my pop and my candy. So yes, I take both armrests and nobody can say a ****ing thing about it!
Hmm. I think I might be an exception to 'the rule' or a special case. I haven't been to the movies in a long ass time, but I can clearly remember that I never used the arm rests. I like to rest my arms firmly on my lap, preferably with my hands clenched together. This has always been my ideal sitting position, no matter where I'm sitting at the time. However, I would assume that the arm rest to the right 'belongs to' whoever's sitting in the seat or whatever, based on @Batman's post above. Then again, it seems to all boil down to common courtesy and good judgment between the two adjacent sitters.