If you get interference simply from installing a second unit on the boat ... It would most likely be crosstalk interference from the Sonar beams overlapping (like Catfan mentioned) ...

Crosstalk interference can sometimes be controlled with the Noise Filter under the Sonar tab ...

If the Noise Filter will not eliminate the crosstalk ... Turning the Sonar off 1 unit or sharing the same xducer thru Ethernet-ed network settings is the best remedy ...

Rickie