Both sides of the tacho 'loop' go to the same black/white wire.

There is a distance along the wire between the points that these wires are attached to the b/w wire. The points that they join the b/w wire are between the ignition switch and the ballast resistor on the coil.
The b/w wire is the 'ballast resistor 12v' wire from the ignition switch.

Alternatively, you can 'cut' the b/w wire , and put the tacho inline with the b/w wire. Don't crimp the connectors on, until you figure out which direction the pulses need to go through the 'loop'.

eg.

Ignition switch(IGN terminal)---b/w---tacho connector---tacho---tacho connector---ballast resistor---positive on coil.