I have two finders on my boat and run them at different frequencies, they work fine that way. If both of yours run at say 200khz you will get interference because both will receive the signal of the other unless they are far apart on the boat. In fact, you can receive the return signal from other boats if you are fishing close enough to them. A better situation is to have two displays and one transducer. Or as I said, run them at different frequencies. On a Lowrance, you can check the "stop chart" feature to shut off the transducer without shutting off the unit while using the other one.