I fond this search the web
Each entire CAN bus can only be powered by one engine on multiple engine boats. If you locate the CAN bus connector on your engine (the one with the yellow TERMINATOR cap, that you will connect the smartcraft cable to), about 3 inches below it (3 inches along the harness) there should be a 3 wire plug
(this is the CAN BUS power connector), it should be red/blk, blk, ppl. this needs to be disconnected from ONE of your engines and caped (not the starboard engine).
and also this
Engine Gateway Installation
Certain dual engine setups share data between both engines through a single
wire. The associated connector must be unplugged to prevent data collisions while using Engine Gateways. Some applications use a three-wire connector, others use a single-wire connector. These engine setups designate one engine as master and the other as slave. It is common for the starboard engine to be master and the port as slave, but is not necessarily true for all applications. The wiring
harness on each engine may have a wire with labeling stating that it is to be grounded on the slave engine and left disconnected on the master engine. If there is no common data wire or master/slave grounding wire, than each engine will respond as master.