  The angular distribution of the phase space arising in two-particle emission
reactions induced by electrons and neutrinos is computed in the laboratory
(Lab) system by boosting the isotropic distribution in the center of mass (CM)
system used in Monte Carlo generators. The Lab distribution has a singularity
for some angular values, coming from the Jacobian of the angular transformation
between CM and Lab systems. We recover the formula we obtained in a previous
calculation for the Lab angular distribution. This is in accordance with the
Monte Carlo method used to generate two-particle events for neutrino
scattering\cite{Sob12}. Inversely, by performing the transformation to the CM
system, it can be shown that the phase-space function, which is proportional to
the two particle-two hole (2p-2h) hadronic tensor for a constant current
operator, can be computed analytically in the frozen nucleon approximation, if
Pauli blocking is absent. The results in the CM frame confirm our previous work
done using an alternative approach in the Lab frame. The possibilities of using
this method to compute the hadronic tensor by a boost to the CM system are
analyzed.
