ANY port that is shared will have a reduction in power. Your power supply puts out "X" amount of power. The first item plugged into it will receive ALL of that power, less the loss of conversion and transmission. When you plug in a SECOND item, the amount of power from the power supply remains the same, with the exception of the power loss by wire resistance due to the increased current. That is why a higher output charger comes with the "Smart" phone than comes with the older "Flip" phones. If the device is in "reduced power mode" there is more current available to charge the battery than there would be if you were using the phone actively. The voltage remains very close to the same - there is "SOME" voltage loss due to the resistance of the wires with a higher current load. The problem lies with the CURRENT output capability of the power supply, that is the limiting factor.