#1..What would sound better, 2 10's or 1 12"? i know the 10's would probably be louder but a 12 gets lower and ive heard sounds better.
#2..Whats a good shallow 12" sub to fit under the back seat?

 

if you dont plan to raise your seat, i would look at getting a pair of 10s. lifting your back seat will open up a lot more options and a pair of 12s is easy to fit, plus you will have more airspace so your subs can play louder.

as far as what will be louder, it depends on so many things including what sub model, enclosure type, enclosure build, enclosure airspace, power, etc. you can have a 12 that will play louder then two tens: or two tens that drop lower then a 12. many different factors.