I would advise against this design....never face subs towards each other, will sound like shit. The sound waves hit each other and canel each other out giving you a crap sounding bass.
R33's dont have a very big interior, 1x12 will sound awesome.
i prefer 2x10's, nice tight bass and quick response time for listening to drum n bass, dance etc.
And anyway, who wants to be one of those tools who drives down the road, whole car rattling and all you can hear is bass. DUnno about you, but i like hearing the music aswell.