The porno skit sounds like a reprise of a scene that they did in "Things are tough all over" (1982). The film was mostly complete garbage, but their reactions in the porno theater scene were hilarious. It was just the other 80 minutes or so that was brutal to watch. It's worth a cheap rental for that one scene alone.
Edit: I should add.....assuming it is available on DVD. If I can find this film as a rental I may take a look at it. I'm curious how the skit you described would compare to what they did in the film.