I tried this once too, before I understood why it doesn't work. The impedance of a second pickup changes the resonance of the primary pickup, in some way that is audibly different. The dummy coil has to be designed in a way that it generates maximum noise voltage while not also loading the pickup. The dummy coil can be wired in series or parallel with the primary pickup, either way it loads the pickup. But the dummy coils with a wide area are able to produce just as much voltage without the high impededence, so they're more invisible to the primary pickup. They will still impact the primary coil to some degree, so you might still hear a difference, but it shouldn't be half as bad as using a second pickup as the dummy.