[BIG_D]

What type of synchronizer is installed?  Likely a Sturdy or Glendinning.  Regardless, it needs tach inputs from both engines and that probably got screwed up when you swapped stuff around for the Garmin interfacing.

The unit is a Glendenning. I traced the problem to the switch on the dash. That has been replaced and you can audibly hear the Glendenning engage now. However, upon sea trial, the Glendenning engaged , but only the SB engine engaged with the single SB throttle advancing, but the Port (Slave) engine did not throttle up...