  In double parton interactions, the two hard proceses are correlated via
double parton densities. We examine the double Drell-Yan process and the impact
of such correlations on the differential cross section. In particular
correlations between the spins of the interacting quarks can induce
correlations between the decay planes of the vector bosons. We investigate
upper limits on spin correlations following from positivity of the double
parton densities.
