Facebook makes find screening insanely easy. Log onto your Facebook account and search for a group called sneak screening in Boston. The group routinely updates the group with passes for screening around town. Simply print out the pass for two and go see the movie.
College campuses are a great way to get passes for screenings. Luckily, Boston has a ton of colleges in and out of the city, so peruse the college bookstores and the student unions. You will typically find passes for movies lying around free for the taking. If you don't see any, just go up to the counter and ask if they have received any recently.
Another great place to look for passes is at the public libraries. Boston's central library in Copley Square is the oldest in the country, but passes are also available at all the branch libraries in the surrounding suburbs. Also, look in the free magazines and dailies around town. They often advertise free screenings or opportunities to win free passes.
Use the Internet. Many websites such as wildaboutmovies.com and even ew.com allow you to print out passes for free movies. You can search by movie or by city. You can do a simple Google search to find even more sites.