  A search for a hypothetical heavy scalar particle, $X$, decaying into a
singlet scalar particle, $S$, and a Standard Model Higgs boson, $H$, using 140
fb$^{-1}$ of proton-proton collision data at the centre-of-mass energy of 13
TeV recorded with the ATLAS detector at the LHC is presented. The explored mass
range is $300 \leq m_X \leq 1000$ GeV and $170 \leq m_S \leq 500$ GeV. The
signature of this search is one or two leptons ($e$ or $\mu$) from the decay of
vector bosons originating from the $S$ particle, $S \rightarrow
W^{\pm}W^{\mp}/ZZ$, and two photons from the Higgs boson decay, $H\rightarrow
\gamma\gamma$. No significant excess is observed above the expected Standard
Model background. The observed (expected) upper limits at the 95% confidence
level on the cross-section for $gg\rightarrow X\rightarrow SH$, assuming the
same $S\rightarrow WW/ZZ$ branching ratios as for a SM-like heavy Higgs boson,
are between 530 (800) fb and 120 (170) fb.
