  We study the turbulent evolution originated from a system subjected to a
Rayleigh-Taylor instability with a double density at high resolution in a 2
dimensional geometry using a highly optimized thermal Lattice Boltzmann code
for GPUs. The novelty of our investigation stems from the initial condition,
given by the superposition of three layers with three different densities,
leading to the development of two Rayleigh-Taylor fronts that expand upward and
downward and collide in the middle of the cell. By using high resolution
numerical data we highlight the effects induced by the collision of the two
turbulent fronts in the long time asymptotic regime. We also provide details on
the optimized Lattice-Boltzmann code that we have run on a cluster of GPUs
