if your planning on running multiple speaker and amp setups invest in an active crossover of some sort, will be the best thing you will ever buy.

the car sound system is no differant to a real outdoor systema nd you would never see a sound enginner just connect a set of speaker to an amp without a mixing desk, this is simply what an active cross over does.
I play in a live rock band, and have 10 inputs into my desk(crossover) then out to over 2.5Kw of sound, you can then EQ the sounds to the correct speakers and remove the frequencies that you do not want, e.g no bass to tweeters, and no high to subs, without a mixer(desk/active crossover) you cannot do this