Don,
You do need a second ground path, it does not need to be a good one though.
Current will flow along the conduit and needs a path back. This is why we can perfecly safely use single conductor TEK as long as you gorund only ONE end of the sheild.
In the case of this setup, the second conduit would be the return path, because the primary currents are 180 deg out of phase, the induced currets would also be.
