I've looked into this recently and if you want to share data between the two (for example display the sounder connected to one unit on the other screen), I believe they both have to be HD units. You also need an ethernet hub and the appropriate cables. This past summer bought a Gen2 HDS5 as a backup for my Gen1 HDS8. I haven't yet hooked them together and I'm not sure I will. I'm wiring in a second sonar transducer so I can use either as a fish finder and I'll have backup in case a knock one transducer off running over a log. I have enough slack in the radar cable that I can move it between units if one craps out. I did the same for the transducer cables. I did recently buy the Point1 heading sensor that I need to get radar overlay on the larger screen so I'll be hooking that up in the next week or two. Each HDS is fed AIS signal from a separate VHF/AIS radio (Std. Horizon 2150) so with all the redundancy I have, I didn't feel a need to network the two heads together.