AHHHaaaa. A Reader of the Manual! GREAT. Smart. But don't hesitate to ask questions. You already have again so here's some talk back...
If you will be using just one antenna and sharing it between the two 520c units, the best location is.......
well, it depends on which unit you will use most to set waypoints. I set and follow routes from the console unit, and set waypoints on the fly, like when running over a big shad school.
Most of my fishing time is under trolling motor power, so I want full functionality at the bow. When i set a waypoint, say a hot stump, I want that to be very accurate so I don't run over it next time. I put an antenna right over where the trolling motor transducer sits in the water. That gives me pinpoint position accuracy of what I see with sonar. I could get by with that antenna shared at the console, but then the GPS position would always be 17 feet off from what the transom transducer shows me on sonar. Lots of folks accept that. But I wanted pinpoint accuracy at the console too, so installed a second antenna right above the transom transducer.
While at it I put a transducer splitter at the bow, a switch that allows me to use a backup transducer, a second one on the transom. I can now get a second look at a target while trolling over it. That helps me know whether fish are moving off target.
Jim