Back to the topic at hand, retrolefty gave you the solution if what you describe has to happen. In many ways that solution is similar to what you do when you have a battery connected as a back-up power supply. The two USB would get connected like the regulator and batter in the following circuit with a pair of low Vf diodes.
Which ever of the two has the higher output voltage 'shuts off' the diode of the other and the device with the higher voltage provide ALL of the power to the circuit in question.
