Will only using one sub instead of two use less watts?

It will absolutely use wattage but even if your amp was pushing 1200 watts RMS which i don't see, you should only have some "dimming" issues you should be able to drive still as that's when your alternator should be keeping your battery full(est), .... my suggestion would be to make your way over to ebay & find yourself a 1 or 2 farad capacitor.that should do the trick! if you don't want to go that route another option (requires a bit more paper) is to upgrade your charging system, that includes but not limited to your battery, POS & NEG battery.cables alternator... hope that helps!!