The Kissbox USB pass-through is a pseudo-pass-through; probably the same with the Dura C pass-through. The in-line battery is supplying power to the atomizer, not the voltage from the USB port. The USB is only keeping the in-line battery charged. So, the 5volts coming from the Kensington PPP is not getting through to your Durac C, just the 3.7v or so from the in-line battery.