I have a Garmin unit with transom mounted sensor. I want to install a second unit on the bow (TM mounted).

How likely would interference be if a brand other than Garmin be?

Any unit with the same frequency will cross talk when the cones overlap. It's no problem in shallow water though unless your boat is real short.

I since called Gamin. The guy said there needs to be 10 ft separating the 2 sensors if on the same freq.

So bow and stern mounts should be okay.

Anybody find this not true?

I used to have a pair of Garmin 160's on the bow and stern of my 18 1/2' Champion.  Without fail, at least twice a day the 2 units would begin to sync up, cause interference on the screen (usually dashed verticle lines on the screen) and then lock up.  I would have to disconnect the power to one or both of the units to unlock them, replug them back in and then they were good to go.  You can sometimes adjust the ping speed on one in order to prevent this.  Sometimes it works, sometimes not.