For my 18R-G (with Haltech E6X) I'm planning to use a Ford-style 36-1 tooth wheel mounted behind the crank pulley. You can buy the wheel from Motec (look up the catalog on their web site) then you get the centre hole drilled out to match your crank. Then grab a hall effect sensor from whatever and off you go.
I have a feeling that a wheel with two teeth isn't going to have very good accuracy.
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