  We describe completely 2-solitary waves related to the ground state of the
nonlinear damped Klein-Gordon equation \begin{equation*}
\partial_{tt}u+2\alpha\partial_{t}u-\Delta u+u-|u|^{p-1}u=0 \end{equation*} on
$\bf R^N$, for $1\leq N\leq 5$ and energy subcritical exponents $p>2$. The
description is twofold.
  First, we prove that 2-solitary waves with same sign do not exist. Second, we
construct and classify the full family of 2-solitary waves in the case of
opposite signs. Close to the sum of two remote solitary waves, it turns out
that only the components of the initial data in the unstable direction of each
ground state are relevant in the large time asymptotic behavior of the
solution. In particular, we show that $2$-solitary waves have a universal
behavior: the distance between the solitary waves is asymptotic to $\log t$ as
$t\to \infty$. This behavior is due to damping of the initial data combined
with strong interactions between the solitary waves.
