I agree that Ethernet gives the best solution. Your units will synch waypoints even if one unit has been off or in low-power mode, not son with NMEA 2000. NMEA 2000 also won't delete waypoints from the 2nd HDS unit if I understand operation correctly. But, if connected via Ethernet when you delete a waypoint, it will also get deleted on the other HDS.
A little tidbit for waypoint synchronization: If you want to delete a waypoint, make sure all the units are powered up and on the Ethernet network when you do the delete. That will get rid of it on all units. If you do NOT delete in such a manner, when the units are powered up, the one still with the waypoint will restore it on the HDS where you did the manual delete. You'll see the deleted waypoint reappear.