Why don't you just sell one of the subs, and buy another matching one so you have two of the same? But on another note, i don't see the problem with that, as long as they are in separate boxes, look at tech force services running 2 18s, and 2 15s.
i guess it depends on the vehicle, i have a few friends where you its louder once you open the windows, hell when dc derrick had his windows open it was 2x louder than sealed specially going down the road
I would go to a civic, and i drive a 92 jeep cherokee, i burn bout a 1/8 gas going to school and home a day. And that is babying it, staying below 2 grand.