2 different sized subs

ok im going to ask a question posed by many a noob. but lets throw all preconceived notions out the window and actually think about this one. i have no real desire to do this by the way, just bored at work. no cars to work on and too much time on my hands. so lets say you want to have 2 subs, of different sizes. not like the stupid kid that thinks more is better and louder, im talking about actually tuning them like a sub componenet system. what prompted this was finally porting my previously sealed box that was too big to be sealed, for my solobaric l7. i tuned it to 28hz and its now ten times louder than it used to be which equals a happier me. but i now notice very distinctly, the difference in volume from the lowest(loud) to the not quite as low(still loud tho). so this got me thinking, what if you had a good 12 or even 15 tuned super low like mine, and complimented it with a 10. i would guess you could have a bandbass filter on the 10, or just put it in a bandpass box, or just simply tune it to (and i have no idea really) but lets say 80hz. i know having 2 different 12's for example actually counteract each other because they aren't moving the same speed or have the same xmax so its like they are partially out of phase and may be better of just having the one, but im theorizing that the 10 with a high sloped HPF, would be relativly silent while the 12/15 was down in the low doin its thing, then the opposite if the bigger had a high sloped LPF on it. so i figure all the best interior speakers utilize specific crossovers and varying woofer size. why couldnt subs benefit from this. but anyway, this is just something to exercise the brain and all your technical knowledge, im open to both yes and no answers but lets try to keep the flaming to a minimum if we can.

Theoretically possible, and effective. However. impractical and very difficult. Theoretically you can have 1000 speakers playing just a small range of frequencies to give you a "full range" playback. The Kicker L7 is a monster of a woofer design for SPL. Yes its supposed to play louder at lower frequencies. If Mib-bass is what you are looking for. . . there are other options besides kicker.

"what if you had a good 12 or even 15 tuned super low like mine, and complimented it with a 10"

Keep in mind that the tuning freq describes the box function, not the sub.  The sub is still engaged with the bandwidth that the crossover provides it.  If you are using very specific crossover points, you are not talking about two subs, but are describing a sub, a bass, and mids.  In reality, your sub (no matter what it is tuned to), should play 80 Hz equally as well as a  bandpassed 10 could play the range.

The difference with subs over bass and midbass woofers is the limitation of the octave range.  With subs, you're going from 20 Hz to 80Hz, two octaves (if you've got good enough gear to hit that lowest octave).  With most systems, it's ONE octave. 

Contrast that to the higher bass frequencies and you see what I mean.  Bass picks up at about 80 Hz and goes to about 600 Hz, mids from there to tweeter range at 3K or 4KHz.  That's like 3 octaves for each of them, and most systems have one set of speakers responsible for the entire 5+ db range.

That's the biggest reason you have to decide what your preferences are when you select a sub system.  Is it going to be SPL or SQ, and isn't that what we always ask?  Do you want musical range or do you want to be the loudest system on the street?  There's a system for all tastes, but there's no one system that pleases all tastes.

Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.

now thats what im talkin about, i love the complex answers i get here. now my understanding is that tuning a box to x hz means that is going to be loudest and/or most efficient at that freq. (correct me if im wrong) so my thought at first, was to have on tuned to lets say 25, but that would make it lack in the range closer to 60-80. so then have one tuned to 80, which would lack in the range of 20. now my problem is that i dont really have a grasp of what number to assign to a freq that im hearing, so maybe my numbers are way off. my idea is that the higher freqs that my sub is hitting, that are still loudish but not as loud as the really gutsy ones, are around 80-100hz (again please correct or clarify this) maybe my sub is doing more than i give it credit for and i am really talking about freqs upwards of 600hz. i really dont know. i guess if i am then the answer would be some nice 8" mid-bass drivers. but anyway with your explanation im thinking that box tuning, while still probably a factor, would pale in comparison to some steep crossover points. i guess what i really need to know is, what a certain freq really sounds like. ill try to throw an example out there, the long sustained notes you find in rap music are probably pretty close to 20. so what about those songs that have a good solid beat that just doesnt make your sub punch quite like rap, maybe a techno beat. you know the songs that you think "this should be hitting gard but it seems too fast or just above my subs sweet spot, like it should be down there, and loud, but isnt. again i feel thats somewhere in the 100-150 range. is that actually much higher? anyway its late and im rambling... continue discussion.

and again id like to reiterate, that when i say me, its a hypothetical "me". this is really just for educational purpose, with my own car thrown in there to make it easier for me to understand. so throw things like practical and feasible out the window, i just wanna learn a little more about speaker dynamics here.

hmm... Most of us at some point get something what I call "hearing Block" This is when we try to assign a specific sound to a particular frequency. You mention 20hz. . . lemme tell you buddy i doubt you can physically "hear" 20 hz. So, back to the mention, trying to analyze the different bass notes will only lead you to believe that you are "missing" bass. It happends to all of us. Yes, the port tuning increases the output at, and around, that tuned frequency. Have you tired the WINSD program? READ HERE This program gives you a visual of what the L7 "should" be doing. Its nice and the tutorial is easy to learn. I have two kicker CVX's on the wifes car and i was able to get a clean 35hz response with a vented 3 cubic ft box.

so after your responses and continued research i think i have a better understanding. i guess it really boils down to some mid bass drivers. so the question is, are they worth it? i had a hummer, whose owner had more dollar than sense/cents (kinda looses it meaning when you type it). but he had everything in there, avic Z2, 2 15" overheads, 4 headrest monitors... you get the idea. but he also had 8" in the q-logic kick panel boxes. now this thing sounded like crap because he didnt know how to work the head unit and wouldnt let me adjust his settings, but they seemed to have a good bit of punch to em when i turned off his 13" w7. so thats the question really, will i notice a diff if i already have a pretty good sub. my setup is way more SQ than SPL, so the sub isnt drowning everything out. or would a set of 6x9s in boxes provide pretty much the same effect.

high SQ takes a lot more than doping a sub in the trunk. So don't feel discourage it takes a lot of time to get it just right. . . but when you do, oh man its the doodie!