To get two you have to buy two, no remote screens available from HB. IMO the best route is sticking to "same" units. SI with a SI, DI with a DI, etc. What I did. 898 on the console and a 798 on the bow. Wish they were both 1198s but how it goes $$$ wise. What is nice about going this route is if you connect them ethernet wise you can share the transducers. So If I am on the bow I will change the consoles transducer in the ethernet settings to the bows transducer so I don't get any cross chatter between them for my daughter in the rear of the boat. If we are out chasing Striper and trolling with the gas motor, etc I will change the bow to the consoles transducer on the transom. That way I can set the front unit to whatever screen and use the consoles for other views. I just have to look over the windshield for the bow units view.
Unless I am out of date on products there isn't a 500 or 600 unit with Ethernet. What you have to look for if you want to be able to share waypoints, etc. 700HD units and up are all ethernet capable.