You can use two amps to power 1 DVC sub.
It's hard to say how much you'd gain by using two amps at 600 watts vs 1 at 200. You wouldn't gain anything if you don't need more than 200 watts to power the sub for your system. Plus that Kicker sub may be rated to handle 400 watts but that doesn't mean it continues to get louder as more power is applied. It may reach it's maximum output at 250 watts or 300 maybe 150 watts, unless it was tested who knows really.
Beware of anyone saying you'll get 4.5db's louder because that isn't a given at the power level you are talking about.