  We present a calculation of the effective cross section $\sigma_{eff}$, an
important ingredient in the description of double parton scattering in
proton-proton collisions. Our theoretical approach makes use of a Light-Front
quark model as framework to calculate the double parton distribution functions
at low-resolution scale. QCD evolution is implemented to reach the experimental
scale. The obtained $\sigma_{eff}$, when averaged over the longitudinal
momentum fractions of the interacting partons, $x_i$, is consistent with the
present experimental scenario. However the result of the complete calculation
shows a dependence of $\sigma_{eff}$ on $x_i$, a feature not easily seen in the
available data, probably because of their low accuracy. Measurements of
$\sigma_{eff}$ in restricted $x_i$ regions are addressed to obtain indications
on double parton correlations, a novel and interesting aspect of the three
dimensional structure of the nucleon.
