This article deals with the derivation of a fundamental theory for describing mass transport on the active surface of a two-strip mass transfer probe for an arbitrary direction of the fluid flow. The existence and finding of such a general theory is a critical point for using the electrodiffusion method by employing twostrip probes. Considering changeable probe geometry, the general analytical formulas for the average mass transfer coefficients are derived and provided for both the probe segments in dimensionless forms. The correctness of the derived analytical expressions is confirmed by numerical solutions of the convectiondiffusion transport equation. Also, a methodology of possible experimental data treatment is proposed. It is based on the evaluation of two current signals collected from the segments of a two-strip probe. From the derived equations, it is possible to determine the magnitude and direction of the wall shear rate for both, predominantly frontal and also reversal flows. Established on the analysis of the current ratio predictions for various probe geometries, an optimal probe configuration is found out with respect to the sensitivity of flow direction measurement.