Since I got my boat a few years ago I always wanted a tachometer, the boat had never been fitted with one. I was very reticent to spend a bag of $ on a new yanmar flywheel tach sensor (#124770-91162), but luckily scored one cheaply off eBay. Yes, you can still once in a while buy something cheaply from eBay.
The sensor has two identical connectors on it. I would have expected only one, as I imagined it would give off positive pulses, and the negative would come from the common ground of the boat. The tack of course has only one input and a ground. I gather this sensor is meant to be able to feed two tachometers?