If one has an electric oven, stove, dryer, etc, I think all these appliances use both legs of the circuit (220 volts in the U.S.) and should act as a bridge.

Another thing you might try to see if a coupler is working is to move a maxi or mini to different outlets to see if the device gets a signal.

You should be able to determine which outlets and switches are on the same leg by turning off some circuit breakers.

I think electricians in new construction should provide a wiring schematic but they often (usually/never) don't.

/* rant mode off */

-Jim.