No, the same “puck” (GPS Receiver), transducers, power cable, mounting bracket and cable holder all are the same between the 898 and 998 HD units.
When Ethernet connected and you mark a waypoint with the front unit it will use the coordinates of the GPS Receiver that is connected to the front unit. preferably the GPS Receiver for the front unit is mounted close to the transducer that the front unit is using as well. The 998 unit will have the same coordinates as a reference even though it is using a different GPS Receiver.
Each unit having its won GPS Receiver, with that GPS Receiver being mounted as close to directly over the transducer that it is using will make for the most accurate waypoints.