Yeah, Humminbird has a Interlink option for $100, but it only allows you to share waypoints, tracks, and routes. You can get transducer spliter-cables and run multiple units off one transducer, or hook up a high-speed trans. to your 798 along w/ the side imaging unit. Lowrance does have a linking option (Navico Expansion Port), but is a lot more expensive, and cables to connect to it are seperate :/ . But no, you CANNOT see Navionics or any other maps on an interlinked unit if it does not have the card plugged into it. One chip per unit kinda deal. However, if you are looking for replacing both fishfinders and saving a few $$$$, look at getting a 798 c SI (External GPS) and a cheaper 700 series, like a 778c. You can run 2 units off one GPS Module as long as they're interlinked (Lowrance doesn't offer this as far as I know).