I think there may be a problem with this approach. Many 3D blu-ray sets include a 2D version of the movie on a blu-ray disc, and a separate blu-ray disc which contains only the 3D version of the movie.
Many 3D versions are mastered differently...specifically, the brightness on a 3D blu-ray may be boosted in order to compensate for the reduced brightness that happens when wearing 3D glasses. If you use a 3D blu-ray and turn 3D off on the projector, yes, you're seeing a 2D image but it's not going to look the same as the native 2D blu-ray disc would look. Brightness may be different, and the "half" of the 3D view you're seeing may look different in other ways (depending on how the 3D effects are rendered).