I had the Humminbird 798 Ci si mounted at the console with its SI transducer mounted on the transom and a shoot thru the hull mounted in the bottom. I'm installing the 898c si on the console. Will the 898 work with the 798 transducers or do I need to change them out?
Next, I'm going to mount the 798 on the bow so what do I need to hook the graphs together to use the transom mounted SI transducer with both units?

The transducers are different. The 798 comes with the standard Compact SI transducer and the 898 comes with the HDSI transducer.

If your 798 is not the 798ci HD SI, you can only share GPS data with INTERLINK. The HD model is Ethernet capable and so it the 898c SI so you could share a single transducer.

The HDSI is rated for 455 and 800 kHz for Imaging and is longer.
The Compact SI is rated for 455 kHz for Imaging and is shorter.
You would loose imaging quality with the 898 using the Compact SI transducer. The 798 would gain imaging quality using the HDSI transducer.

Thanks Wayne!
So will the HDSI work both at the same time? If so do I need anything particular to make this happen?

What puck would you recommend me to use on the trolling motor for the 798ci si?

I would install the 798c transducer on the Trolling Motor if your current one is working. Get the www.transducershieldandsaver.com With Interlink you cannot share transdcuer data only Ethernet.