OK, maybe some day this will spare someone a half-day of troubleshooting.
Be kind, it's not easy to admit how stupid I was...
Now remember, I spent half the day disconnecting and reconnecting wires, removing and re-mounting sensors and senders, resetting and restarting electronics
, and running the engines again and again.
Before I gave up, one more thought came to me...
I turned off the engine room lights.
Yup. That did it. Both tachs settled right in where they belong, perfectly in sync and moving in the right direction.
Last year I had replaced my power-hog 12V incandescent Edision-base bulbs with florescents. They are SO much more efficient, cooler and give off more light. I was real proud of myself.
At least now I have no phone
calls to make tomorrow. And my next job was waxing, so I'm not THAT heartbroken about not getting to it.