Al brings up an excellent point - one I hadn't thought about. However, he's totally correct. Another thing -- the crossovers inside the speakers are engineered based on the expected impedance of the following driver (i.e. low-pass coil circuit for a single woofer). If you tack on a second speaker in series, the impedance in that frequency area will rise and cause the crossover frequency to move drastically. This will cause the frequency response of the first speaker to have all these bumps or holes - making a very imbalanced sound. That being said, the first entire speaker will also affect the crossover in the second speaker, therefore causing the same problems in that second speaker.
Greenhorn question: Beware!
Is a 4 ohm exciter speaker and a 6 ohm woofer box, wired in series (total 10 ohm) connected to an 8 ohm output amp OK?