  We investigate the non-equilibrium dynamics of a one-dimensional spin-1/2 XXZ
model at zero-temperature in the regime $|\Delta|< 1$, initially prepared in a
product state with two domain walls i.e,
$|\downarrow\dots\downarrow\uparrow\dots\uparrow\downarrow\dots\downarrow\rangle$.
At early times, the two domain walls evolve independently and only after a
calculable time a non-trivial interplay between the two emerges and results in
the occurrence of a split Fermi sea. For $\Delta=0$, we derive exact asymptotic
results for the magnetization and the spin current by using a semi-classical
Wigner function approach, and we exactly determine the spreading of
entanglement entropy exploiting the recently developed tools of quantum
fluctuating hydrodynamics. In the interacting case, we analytically solve the
Generalized Hydrodynamics equation providing exact expressions for the
conserved quantities. We display some numerical results for the entanglement
entropy also in the interacting case and we propose a conjecture for its
asymptotic value.
