If you use 10A to charge a 2S LiPO pack, maximum power required is 8.4 x 10 = 84W
Charging two pack together = 168W, total input power will be < 200W. So the maximum power supply current will be 200/12 = 16A.
I just use a 12V/20A power supply, so I set the power supply current to 20A only. The charger can set % currenr share between the 2 channels, I think this setting is to limit the maximum current per channel.