  We derive exact expressions for luminosities of massive vector-boson pairs
which can be used to describe the cross sections for processes in hadron
collisions or $e^+e^-$ annihilation which proceed via two-vector-boson
scattering. Our approach correctly takes into account the mutual influence of
the emission of one vector boson on the emission of a second one. We show that
only approximately the exact luminosities can be factorized into convolutions
of single-vector-boson distributions. Numerical results are given and compared
to simplified approaches.
