  Using the general helicity formula for $\gamma^* \gamma^*$ collisions, we are
showing that it should be possible to determine a number of independent
``structure functions'', i.e. linear combinations of elements of the two-photon
helicity tensor, through azimuthal correlations in two-body or quasi two-body
reactions induced by the photon-photon interaction, provided certain
experimental conditions are satisfied. Numerical results of our computations
are presented for some particular processes and dynamic models.
