I have recently installed a Garmin 240 blue f/f that uses a dual frequency transducer. The Garmin operates at 50 and 200kHz. Rather than remove my Hummingbird f/f that operates at 200 and 455kHz., I would prefer to keep it as a back up or replace the transducer with a side scanning unit that works at a freq. of 455kHz.
Has anyone tried any of these combinations of transducer frequencies with any success? How much seperation between the different operating freq. do you need to avoid interference?

Juggernaut, I think the only time you might have a problem is when you have both units on the 200 khz frequency, the more powerful unit (Garmin) probably won't be affected much if at all but the Humminbird will most likely not be readable with all interference it picks up from the Garmin.
My own personal experience from the boat I run in Ak, I fish amongst a fleet of 25 Almars all outfitted identically, Furuno 582 ff, when there is a hot bite and most if not all the boats are fishing in tight quarters, I don't even bother looking at the ff, there is so much interference that they become useless, samething happens if I pass close to another resorts boat that happens to be running on the same frequency (I ALWAYS run on dual frequency unless bottom fishing) on the other hand, when I pass close to boats that operate on different frequency or have lower power ff, they do not affect mine.

You know Ak, you brought up something I hadn't thought about. Always wondered why I seemed to get alot more false readings when in close to other boats. But just like my trucks radar detector getting signals from other radar detectors, it makes sense now. Thanks

I have a Humminbird in the back and an Eagle in the front of the boat. The Eagle is 200 khz, as is the Humminbird. I don't get any interference between them. The transducers are mounted two inches away from each other. Don't ask me why. I don't usually run them both at the same time, but it seems to work when I do.