The only time I would not charge both is if I am in my car using a modified sin wave inverter ( 12 vdc to I20v ac) found that all my devices with chargers pluged in to these inverters the charger gets very hot. Since I travel a lot in a vehicle and charge on the go a pure sine wave inverter delivers the same wave form as in your home, they are more expensive but I think it may increase the life of all my device chargers and equipment more sensitive to ac power input.