confusing system setup

hello9, a guy came to me the other day wanting a system installed in his full size 1988 chev truck (extended cab) he wants the seats left in the back, but he's talking about running 2 pairs of subs (2 - 10" and 2 - 12") i'm trying to reason with him to go either 4 10's or spend a little more on the 12", but he seems to be stuck on the 2 different pairs, makeing a proper box for each pair of subs what would the pros and cons be?

now, part two here. the reason he wants to be able to switch between the pairs of subs or have all on at the same time,  mostly for different sound for different occasions. i know i've gotta use relays but i'm not sure where they'd go. and any infor on this would be nice.

here's the setup so far, infinity highs (stock location) infinity amps (one for his highs and one for the subs) i'm planning on running a 2 channel amp and have one 10 and one 12 on each channel so it would be able to run 2 ohm stereo with all speakers wired. and for the subs looking at infinity kapppa perfects 10.1 and 12.1 subs. everyting else has been thought of...i'm just wonding if this setup would work or if my suggestion to him would be better.

the 10's would be in  a sealed box and the 12's would be in a ported box.

What a waste! (unless you are the salesman too). It is just plain ridiculous to have multiple sized woofers in a car, even if they are all not being used at once.  The K.I.S.S method needs to be apllied here.

This guy neeeds to either choose the 10" subs or 12" subs and either a sealed or ported box.

I disagree with this. First of all you can use a 15" woofer in a sq setup if one felt like it. The size of  a woofer is not going to determine the sound quality. And why would you keep the 12"s in a ported box? Why not sealed? 

Again I disagree. I've used that argument in the past, claiming the 15" or 12" is less accurate than an 8" or 10", but that is in fact a very misleading statement. There are many (15") speakers on the market that can successfully compete with a smaller diameter driver. It is dependent on the motor technology, cone structure and enclosure. For example Adire's Xbl^2 tech, very low distortion. My IDQ's are very accurate and they happen to be fifteens.

But why are you suggesting using two different sized woofers at all? 

By the way do you compete, or at least in the competition scene?

I believe that cabinet diffraction response or baffle step is a supression of bass due to enclosure width. Not to mention it's not ususlly applicable in a car.

Props to DYohn