So you are planning to power a pair of those subs with a single 600-700 watt amp? If that is going to be your only amp, I think you will be ok. Just look for a nice class D amp since they tend to be more efficient than a/b amps.

I have a 1999 Integra, I once had a 600 watt JBL bp600.1 AND a Fosgate 600a4 running through my car at the same time. My alternator is still good....for now. There are signs of it getting weak though.