my votes for 2 units also....I can get a real good mark on structure and drop a marker right on it with the trolling motor transducer/ front unit....just got a new unit today for the front that has gps.....so now will have gps on the front and rear.....will be nice to motor close to a way point and be able to get on it a little more stealth with the trolling motor.....2 units works best for me......may not be the correct answer....but like I said thats what works for me.....