I haven't done anything other than play around with the default display to swap around some of the smaller gauge sets. What's strange is that on the Garmin both sides of the gauges are active but it's only coming from one engine (strbrd). It's showing exactly the same oil pressure, coolant temp and RPM's for both. I only know it's strbrd based on revving the engine.
Oddly...the fuel flow rates appear correct on the Garmin. Unless it's simply doubling the strbrd flow rate? That would be odd though...
you said the dip switches are set for port and starboard, right? Try swapping the gateways if it's not too much of a pain, or disconnect one at a time and see if the other one comes online...sounds like maybe they're stepping on each other