Multiple Transducers?

I have a question:

when the guy at the dealership put my new lowrance and transducer in the other day he, he left the old transducer in the boat as well. So now i have two thru the hull transducers, one for the old x48 and one for the new 520c and they are not even a foot away from each other. I always heard that if transducers were too close together and both on at the same time, the signals would interfere. This guy said they wouldnt. I also know that this guy who told me they woldnt interfere has only been in the business for a little less than a year so I am finding it hard to believe that he is correct about them not interfereing. I really dont plan on turning them both on at the same time, but still Id like to know what would happen if I did.