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.