Let's ignore the 2S batteries, since a charger that can handle 4S charging at 2C can handle 2S. 4S = 16.8V fully charged, 6000mah battery charge rate at 2C = 12A. 16.8v x 12 = 201w. If you're doing 2 batteries that's 400w. Your 575w power supply should be more than enough.
As for chargers, you need to decide if you prefer 2 independent chargers or one dual channel one. I used multichannel chargers for years, but after having one channel go bad on two different units, I decided to go back to single chargers.