What Winnie is saying is only true when there is fault current involving the grounding conductor. Normally you wouldn't have any current flowing in the grounding conductor hence no magnetic field. It is important to have the grounded and the grounding conductor as well as the ungrounded conductors in the close proximity ie: in this case the same conduit to maximize conductivity and quick action of the overcurrent device. As someone has pointed out 300.3(B) needs to apply.
