20 amps should be more than enough for you. If your only going to be running two chargers' that would be 12 amps @ amps per charger. the still gives you 8 amps for other product like a pelter cooler, tire warmers and/or discharge tray that require a power source. you might need a distrubution box if your power supply only has one set of banana plug. (there is a set of banana plus that you can plug another on into them there by much more.
Your min would be 12 amps for two chargers. I hope this helps eh.