If you look at the crossover networks in almost any box speaker, youll find that the speakers are run in parallel. Running speakers in series will add harmonics and artifacts to the speakers behind the first driver caused by the inductance and resistance of the moving voice coil of the first driver. If you do this with a tweeter and a large woofer, youll hear the effect. Also, you have no reliable way to control the volume difference between the two speakers since they are not receiving power from the amplifier at the same impedance. Granted, you get the most control running a single speaker from a single channel, but careful planning and understanding of automotive acoustics will allow you to calculate the proper crossover levels to accomplish what you want with a minimum number of amplifier bricks.
Do your speakers a favor to aviod blowing them via a clipped signal, wire them in series. Anyway, The real solution here is a 4 channel amp.