  Adding a scalar singlet provides one of the simplest extensions of the
Standard Model. In this work we briefly review the latest constraints on the
mass and mixing of the new Higgs boson and study its production and decay at
the LHC. We mainly focus on double Higgs production in the $hh \rightarrow b
\bar{b} WW \rightarrow b \bar{b} \ell^+ \nu \ell^- \bar{\nu}$ decay channel.
This decay is found to be efficient in a region of masses of the heavy Higgs
boson of 260 - 500 GeV, so it is complementary to the 4$b$ channel, more
efficient for Higgs bosons having masses greater than 500 GeV. We analyse this
di-leptonic decay channel in detail using kinematic variables such as $M_{\rm
T2}$ and the $M_{\rm T2}$-assisted on-shell reconstruction of invisible
momenta. Using proper cuts, a significance of $\sim$ 3$\sigma$ for 3000
fb$^{-1}$ can be achieved at the 14 TeV LHC for $m_H$ = 260 - 400 GeV if the
mixing is close to its present limit and ${\rm BR}(H \rightarrow hh) \approx
1$. Smaller mixing values would require combining various decay channels in
order to reach a similar significance. The complementarity among $ H
\rightarrow hh$, $ H \rightarrow ZZ$ and $ H \rightarrow WW$ channels is
studied for arbitrary ${\rm BR}(H \rightarrow hh)$ values.
