Many years ago a buddy of mine had a truck with 2 Sound Stream 10 inch subs in it. They shared a common air space and sounded very good. Unfortunatly one of the subs would eventually blow and then you have one sub working in a space twice as big as it is supposed to, along with a "spring" (the blown speaker) that changes the volume of the box. Needless to say the one working sub would stop working and blow in a short amount of time. After 3 sets, the stereo shop put a divider between the subs and he never had a problem.
Personally I prefer to use a divider because of the info above, and also for the fact that not all speakers are the same strength. When I say strength, I am refering to the magnet and the electronic strength created by the amp for the magnet if that makes any sense. Kind of like your right arm being stronger than your left arm because you happen to be right handed. If each speaker is in thier own enclosure, you can be sure the speaker un affected by anything else, but in a shared enclosure each speaker is dependant on the companion speaker.
Make any sense??