Well, you'd need to either actively bandpass them with either an external active crossover, the amplifier's crossover, the HU's crossover, or a combination thereof.
Also, you would need 2 extra amplifier channels to be able to do this.
But, you can pick up some OZ audio, Rainbow, or DLS component sets for under $150 for the pair and would give much better overall performance than what you are trying to do.