If you have an overlap that is causing the signals to cancel each other out you can turn down the transmission range so that they are no longer overlapping. This can be set to a radius of 2-10 ft. With that being said, if you can put one of the transmitters in the middle of the 15 x 18 area it will be able to cover the entire space. The transmitter can cover 10 feet in each direction or a 20 foot diameter.