  Colliding collisionless shocks appear in a great variety of astrophysical
phenomena and are thought to be possible sources of particle acceleration in
the Universe. To investigate the detailed dynamics of this phenomenon, we have
performed a dedicated laboratory experiment. We have generated two
counter-streaming subcritical collisionless magnetized shocks by irradiating
two teflon (CF$_2$) targets with 100 J, 1 ns laser beams on the LULI2000 laser
facility. The interaction region between the plasma flows was pre-filled with a
low density background hydrogen plasma and initialized with an externally
applied homogeneous magnetic field perpendicular to the shocks. We report here
on measurements of the plasma density and temperature during the formation of
the supercritical shocks, their transition to subcritical, and final
interpenetration. We have also modeled the macroscopic evolution of the system
via hydrodynamic simulations and the microphysics at play during the
interaction via Particle-In-Cell simulations. The main goal was to understand
what was the effect of the second shock on particle energization. We found that
in the presence of two shocks the ambient ions reach energies around 1.5 times
of the ones obtained with single shocks. Both the presence of the downstream
zone of the second shock and of the downstream zone common for the two shocks
play a role in the different energization: the characteristics of the
perpendicular electric fields in the two areas allow, indeed, certain particles
to keep being accelerated or to avoid being decelerated.
