As was mentioned before, it's most likely limited by the power supply (voltage and current). For example, I run my 406duo off a 12v/63amp power supply. The 406duo is limited to 60A input, so I can only get about 720 watts of charging power. Half of it's rated 1400 watts. To get full potential I'd need a 24v power supply. But I don't need it.

Quick math is: # cells * 4.2v (assuming regular lipo not HV) * charge amp rate = charge power in watts. If you're doing 2 batteries at same time, multiply that number by 2. That's the min wattage needed. But not all power supplies are created equal. Some have pretty bad voltage sag as you get close to their rated capacity.