[quote name='Bill Fitzmaurice' timestamp='1471876703' post='3116607']
Technically they don't both emit sound. The area of only one cone is exposed to the air, so the maximum output with two drivers isobaric is the same as with one in a standard configuration. The only advantage is a halving of the Vas, but due to the space taken up by the second driver and the connecting plenum chamber the net cabinet size is not halved, while cost is greatly increased by using two drivers. That's why manufacturers in general have not jumped onto this particular band wagon.
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That's not quite what I meant Bill, I meant the rear-facing speaker is also open to the air, just facing backwards, or even upwards or downwards.