1197c Si to 1197c Si Ethernet connection is easy: just the one cable connected to the Ethernet connectors of both units. Which AS-EC-XXE cable you will need for this depends on how much cable you need as they come in 2, 10, 20 and 30 foot lengths: AS-EC-2E, AS-EC-10E, AS-EC-20E, and AS-EC-30E.
If you are going to use only one GPS Receiver than you just connect the GPS Receiver to whichever unit you will be leaving on all the time (this unit has to be on for the second unit to use the GPS Receiver as it uses it through the first unit).
A better option would be to connect a GPS Receiver to each 1197 unit as this will give you better waypoint accuracy (if the GPS Receivers are mounted close to the transducer that 1197 unit will be using). It will also give you a second GPS Receiver in case one goes out during a fishing trip.