Just out of curiosity, any reason people are running the twin speakers as opposed to a single?
The main reason to use two speakers would be if you have a stereo radio and want to retain some semblance of stereo quality. If you're using the original mono AM or mono FM radio the original style single speaker would be a great choice. If the radio you want to use has stereo outputs and you don't want to cut holes for speakers or have unusual speaker boxes showing in your car the twin speakers in the original dash opening is an option. It actually provides reasonable stereo seperation reflecting off of the windshield. Not real high quality, but reasonable. They sound fairly good with some decent coaxial drivers and blocking capacitors to eliminate over driving the small cones. A hidden subwoofer can be utilized to augment those bass notes too.