2 car batteries?

Ohm's Law.  Do you know what that is?  How about Kirchoff's current law?

  Go ahead and install the equipment you want...Amp,Subs,Etc...Make sure you do "The Big 3" upgrade before you install the amp....(just do a forum search on the big 3) It looks like this might get a lettle ugly...! So post what equipment your defitnely installing... (Are you going to install this stuff yourself...Or have a shop do it...?) Once you install the equipment and properly tune/set gains,and do THE BIG 3 upgrade...Lets get a meter on there and see what your voltage is dropping to...Have you put any thought into the enclosure/box the subs are going in...? Lets also make sure you are installing the correct size power wire...And the point at which you ground the amp is soooooooooooo important!!!!!!!!! My advice is to make a decision on the equipment...But dont cheat yourself on the amp kit or box...!

(2) 4 ohm 1" tweeters will draw the same power as (2) 4 ohm 18" subs when hooked up the same way to the same amp.  The amount of power has to do with the overall impedence of the load, not the amp or size / shear number of speakers.

As far as the battery question, like somebody already said if there isn't enough current being generated a HO alternator would be the correct way to go.  However, install your equipment first, do the big 3, and then feel it out.  No use putting money where it isn't needed.

I would recomend a "performance" battery over a stock battery any day - the construction of the battery can determine how efficiently it charges, which can turn more current from the alternator in to stored power instead of heat.

Is this your current set-up...? Are you having any dimming issues...? Or Is this just the set-up your considering...? What amp is it exactly...? Some shops will throw a 2nd battery in b/c thats what the customer wants...Some do it as just an up-sell...For the most part adding a 2nd battery is part of process that should be completed ...Like adding a 2nd battery is step 3 but you missed steps 1-2-4-5...Follow me...? And as Kp said which took me 2 posts and 4 edits...Install...Feel it out...Defitnely upgrade battery under hood...!(also big 3)

Two 12" 4-ohms speakers and two 15" 4-ohms speakers will place the same effective load on an amplifier.  Of course if you change the loading you will change the current draw.  Like I said, Ohm's Law.  It is difficult to know if that's what you are even talking about, however....

First off, more volume = more power so by simply turning the radio up theoretically you will increase current draw.

Second, how many ohms is your 12" sub and what is the ohm load of the 2 15s?  If the 2 15s together have less impedence then the 1 12" sub you will increase power again.