You'll have a REALLY hard time getting two good 12" subs with a powerful enough amp for $700 if you want quality. Those items can be had for a few hundred, but you'll be sorry later. With sound systems, it's best to do it right the FIRST time.
I'd look into one 13w7 from JL with a 1/1000 amp if you can. It'll run a bit more, but REALLY hit.
Otherwise, look at the series 2 or 3 Rockford subs. Make sure you get a powerful enough amp though. I see people get two 12's with a like 400 watt amp all the time.