Indeed. If the same stimulus impacts both hemispheres of the brain at the same strength and at the same time, with the net result that a specific muscle in both forearms contracts, the result would be that both wrists rotate inwards, thanks to the mirror-image layout.
In a roundabout way, I was trying to say that mirror-image behaviour of the wires is the reason that electromagnetism must be thrown out as a theoretical cause/mechanism of detection.
Having said that, in geophysics, it is soil resistivity that senses differences in moisture level at depth. To emulate that, the human body would have to be generating an electical field AND detecting fluctuations impinging upon it. Degree of contact between the feet and the soil would become a factor. Wet grass, rubber boots all kind of variables could interfere with the doswer's success rate.