If the circuit conductors are run in bonded EMT, and the 'quiet ground' conductor is run in EMT, then you have several problems.
1) Because the ground conductor is run separately from the supply conductors, you get 'loop area'. When you run current through a wire, you _always_ produce a magnetic field. The 'strength' of this magnetic field (flux density) is set by the current. But the total quantity of magnetic flux is the product of the flux density and the _area_. The bigger the area, the more total magnetic flux, and the more energy drawn from the circuit to create the magnetic flux. This is _inductance_, and acts to limit current flow. The larger the area, the greater the inductance.
2) Because you've run the conductors in a closed loop of EMT, you get a problem closely related to the issues of running different phases in separate pipes.
What you've done is create a transformer! You have two circuits (closed metallic paths capable of carrying current), closely magnetically coupled (any magnetic field created by one circuit must pass through the other circuit). When current flows in this ground conductor, you will get a corresponding current flowing in the opposite direction through the EMT. The steel of the EMT will also act to increase the inductance of your loop.
3) Finally, if there are any currents flowing in the EMT (the problem that an isolated ground is supposed to avoid), these currents will be transformer coupled to the ground conductor. Thus running this 'quiet ground' in a separate EMT will potentially inject exactly the noise that it is supposed to eliminate.
-Jon
