If this project evolves, I might try making a synchronizer. Until recently the only one was Glendinning which used a tach cable to physically drive screws inside and pull the throttle cable on the slave engine. That and his shore cord reels are really neat contraptions. He has a wild imagination. Yanmar and some others don’t have a tach cable drive so they couldn’t use it. Sturdy makes one now that takes frequency input but they are $$$$$$$.$$. They also make DDEC. One thing holding me back from that is that if it ever failed, the liability could be terrible.