Yep... could interfere for sure.
I have two mounted about 1 1/2' apart and usually run them in auto frequency. Most of the time there is no interference, but if there is a problem you can usually tell when you see a difference in the depth readings. When that happens just manually change the frequency on one and the interference goes away.
I do like running two... one the driver of the boat can use as he wishes... the other one, people fishing can use as they wish!