What cables are needed between the two. I want to eliminate using the StructureScanHD Box and use only the LSS 2 transducer.

I want to share waypoints and have both have SideScan / DownScan on each unit and have separate sonar. I have the regular skimmer transducers for both in place (on TM & Transom) and I guess thats just a setting on each unit as far as separating them per unit.

I know you need the network cable but what about the NMEA2000 cable?

Since you already have a ethernet cord and if its long enough just connected the your 9 and 8.

Thanks Jason
I guess the NMEA2000 cable is for all the engine features plus more

If you decide to add the Point 1 GPS antenna then you will need the NMEA2000 backbone. I run both the Ethernet and NMEA2000 but was able to do what you asked with just the Ethernet. I have the Yamaha hooked up to the NMEA network and see engine data also like you said.

Can you link them together with the NMEA2K? I thought the 8 inch Gen 2 had to have the side scan box?

I believe it does. I had a 9 touch on dash and old 8 on bow and the 8 gets its chart and other data from from the 9. It has a separate trolling motor transducer that you can switch to, but you don't get downscan anymore and it messes with the 9. I had a NEMA 2k and an eternet hook up. I got engine data on the 9 with this set up.

Before you toss the StructureScan box, keep this in mind. You Could put the 9 at the BOW and buy the HDI ( downscan and 2d ) transducer for your trolling motor. This way you have the 8 at the console for sitting down and riding around looking at stuff. When you're fishing, you could use the 9 to see what's directly underneath you vs. what's 20' behind you.. Make sense?

Good thought. I didn't think of that and sold mine.