IS this a good idea, will it sound better/ bigger punch.

I have a 600watt amp so id like to kepp that and run 2 12's off it.

What subs will i be able to use so that they are still being fully powered?

600 RMS or peak power?? 600 peak power isnt much for 2 12" subs, Might wanna get another amp for the second 12". I reckon 2 x 12" subs might sound better though

Well what sorta amps/sub should i be looking at, im not really looks at spending a whole lot.

Or would i be better off just keeping the 15 and working with that, cus all im really looking at is doing a boot install, and i thought 2 12's woild look better than 1 15, but then i think that 2 12s will hit harder and faster so now im looking at them.

Size isnt as important as quality...

2 12's may be worse or better than 1 15.

You usually get what you pay for. 2 12's looks bling though and fills the boot up nicely

I find that 15s are too slow on particular genre's of music, so it all comes down to what the purpose of it is really.

What sorta of 15 do you currently have?

Size is not as important as power and Tuning..
True I do find 12 inchers better for Punch then a 15 inch sub but it really depends on what tuning is done to the enclosure and subs as well as how much power you want to throw at it..

I mean Two IDmax 12 inch subs are going to be better then a $70 15inch Axis Audio sub..

Is you're amp monoblock?
I've got the kicker 600w mono running 2 12" comp vr subs which punch like mikey tyson in a bad mood! :badgrin:

it's doo-able but i spose it all comes down to quality rather than quantity.

Well its a JL Sub, 200watts
A JBL amp 4 chan 600watt.