I was wondering if I can put 2 15's in my cars trunk. Prolly needs 2000-2500 watts to hit hard.Is this too much for my weak lottle civic lol?How can I make it work?! Extra battery?

I don't think your trunk could fit that

first of all...you don't need 2000-2500 watts to hit hard especially in a small car.

secondly yes its way more power than your civics alternator can produce. it's 140 amps for 2k and 173 amps for 2.5k. your car has 70 or 80 amp output alternator...which translates into maybe 40 extra amps of usable power which means at most you can run 600 watts on the stock alternator.

and adding an extra battery doesn't do anything useful for a daily use setup.

So what do you recommend? Just ude one of the 15s? Or I can get an amp?

the sub size is irrelevant to any sort of power ratings. 15s can be 100 watts or 5000 watts.

you need an amp either way so i don't understand what you are asking with "or i can get an amp?".

now to the heart of the matter...

you have 600rms of power to work with so you have a couple options

split the power between a sub amp and a 4 channel amp

or just give the sub the full 600. splitting it will give you better overall results but if you are simply wanting to be loud then option 2 is the obvious choice.

do you have an aftermarket head unit already?

how much are you looking for spend?

idk all that shit sounds hella complicated lol, i guess ill just use one of the 15s instead of both.

why are you wanting to use a 15 at all?

for smashing lows!!!

so i can hear him miles away

It's really not that complicated.

He's saying, your car can produce (in a perfect world with normal conditions) 600W of power, constantly, daily, all the time with no upgrades and having no problems.
If you are really wanting 2k watts rms, you are going to need to upgrade the Big3. Unless you want a soon-to-be dead car. And no, throwing in a second battery or capacitor will do nothing.