Originally Posted by
A.Z.K.
now let me go look into this 'piezo sensors' !
If you were really thinking about making your own, the way to go is probably crank-based. Stages could probably re-range their meter, but I doubt they would want to do that for a one-off. Seems like there are some that use the deflection of the tube between the crank arms to measure torque, of course that's one sided. You could mount strain gauges on crank arms like stages does, but that is getting into significant development issues.
Piezo load cells have the advantage that no motion is required to measure a force. However, in this application I am not sure where you would put them, and the crank arms are being bent anyway, so there is no issue in measuring strains.
As far as your garmin pedals go, I suppose that if each pedal can measure 1kw, then 2 can measure 2kw. In practice, we usually don't put out significant power in both legs at the same time.