We analyze polarization phenomena for elastic lepton-nucleon scattering, parametrizing the $2\\gamma$-exchange contribution, as the product of lepton and nucleon axial vector currents, that correspond to $C=+1$ exchange in the annihilation channel. We found two combinations of polarization observables (one for three T-odd observables and another one - for five T-even observables), which allow to measure the ratio $G_E(Q^2)/G_M(Q^2)$ of nucleon electromagnetic form factors in model independent way, without any specific assumption about the $2\\gamma$-exchange mechanism. Both these combinations have a general character and do not depend on the choice of the spin structure for the $2\\gamma$-exchange. We show the inconsistency of other approximations recently used in the literature.
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