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The use of mode localization phenomenon for mass sensing applications are investigated in this study on two weakly coupled oscillators in forced regime. The motion of such a system is a linear combination of two modes (symmetric and antisymmetric) and mode localization phenomenon occurs when these oscillators are weakly coupled: if a small mass or stiffness perturbation unbalances the system, an amplitude redistribution can be observed. We hereby give an analytic expression of the mass perturbation as a function of the system parameters and the normalized sensitivities. These sensitivities (one for each mode and each mass) have been computed in Matlab and they also depend on the system parameters (coupling, quality factor, eigenfrequency, phase shift between the two excitation forces...). This data is a tool allowing to roughly find out the optimal sizing of such coupled sensing systems (they can be cantilevers, membranes or any kind of oscillators) under given constraints. Finally, the efficiency of this sensing method is demonstrated through a numerical comparison with other sensing methods. This study paves the way for the design of MEMS or NEMS sensors for the detection of gas or biological entities for instance.