Seen this a few times at my local cinema. A group go see one film, when it finishes , you'll see them going into another screen to catch the start of the one just starting in that screen!
... me and a friend do this quite a lot
We find it's all in the timing. You need to go into the second film once all the ads and trailers are underway to stop accidentally sitting in someone's seat.
Did it the other week. paid to see jack reacher and then snuck in to see life of pi 3D afterwards. always have a chuckle to ourselves that they check at least twice to see if the people sitting in the premier seats have a premier ticket and there's us sitting there with no ticket at all
sometimes it's too much to sit through two films at once and we stick with the one. you find yourself dropping off during the second if they are both 2 hours or so