You could use either side (Left or Right) as a Tacho signal.
BUT (big BUT) then the tacho must be setup for 2 cylinders as the sides are only triggered once per rev.

Why? We always used to do it like that (with single coil)? Or because that way you don't need a resistor to 12 volt? Or didn't know the T6 has a tacho output?
Or because they used something else than a T6. You say 37-pin multiplug. Is that the ecu connector? Because a T6 has more pins.