Interference between two sonar units

I run a tiller boat and already have a Garmin 250FF unit hooked up in my electronics locker(transducer at transom obviously). I want to add a full function GPS unit to my boat and noticed that most of them add the sonar capability for another $150. At that price why not get the sonar? No brainer, right? Except that I’m concerned that two sonar units both working off the transom might interfere with each other and mess up the signals. Can/does this happen and if so am I better off just skipping the sonar option? Does it make a difference if you mount the transducers on opposite sides of the motor vs at the same location? Can the sonar be turned on and off as I please on a dual purpose unit? I really don’t need it so long as I continue to use my monochrome Garmin FF250 and that way I’d have a full screen for both. What would you do in this situation? Thanks.

Use the $150 and put it towards a larger screen GPS. Spreading out the ducers will help in shallow water but not in waters over 15 feet. You are able to turn the sonar off but then you just wasted $150

You’ll get some interference anytime you run two units at the same frequency.

I see what Derek is saying, but some people will get a combo unit for a second unit, rather than GPS only. You could get a combo unit without transducer and run it as GPS only. The only reason to do this is if you want to have a “backup sonar” in case your 250 went down while on the water. Just switch the transducer over to the combo unit and you’ve got GPS AND sonar again. Of course, you’d need to get a combo unit that takes the same transducer as the 250 for that plan to work.

Otherwise, like Derek said, just go with the stand-alone GPS unit.