Stacking two identical speakers, with the upper being inverted, gained some popularity in the 70's. You do gain about 6db in (effective) efficiency.
In practice though it does present some challenges. First, the amp is presented a much lower impedance. Often around 2 ohms at some frequencies. Second, the two tweeters are far enough apart that they do not act as a point source over much of the range that they handle which leads to some 'venetian blind' effect.
In practice though it does present some challenges. First, the amp is presented a much lower impedance. Often around 2 ohms at some frequencies. Second, the two tweeters are far enough apart that they do not act as a point source over much of the range that they handle which leads to some 'venetian blind' effect.