tweeters

Wrathchild:  what you're planning on doing simply will not sound good.  If all you want is loud, by all means, go nuts with speakers.  Stop asking us if it will sound good.  It won't. 

If you want your system to sound good focus on ONE driver per channel handling any given frequency range.  (e.g., one left tweeter plays the left highs, one left midrange playing the left mids, one left midbass playing the left midbass, etc.).  If you want loud focus on a mix of efficiency and power handling.  Spend good time and money on the installation.  Expect to drop a lot of money if you want high spl capability and good sound quality. 

Sedate correctly mentioned cancellation as the main reason for not trying what you are contemplating.  Other reasons include phase problems, timing issues, and tonal differences related to differing driver materials and capabilities. 

Look.  If you want loud sound that many would say sounds horrible, go with your plan.  If you want to learn how to do it right, start researching (a) some of the concepts mentioned above (b) speakers and amplifiers that will help you acheive your goal. 

Here's an option if you really want very loud while retaining sound quality.

Horn from ID:   http://www.cardomain.com/item/IDSCD2COMP