  There are only few very-high-energy sources in our Galaxy which might
accelerate particles up to the knee of the cosmic-ray spectrum. To understand
the mechanisms of particle acceleration in these PeVatron candidates,
\textit{Fermi}-LAT and H.E.S.S. observations are essential to characterize
their $\gamma$-ray emission. HESS J1640$-$465 and the PeVatron candidate HESS
J1641$-$463 are two neighboring (\ang[astroang]{0.25}) $\gamma$-ray sources,
spatially coincident with the radio supernova remnants (SNRs) G338.3$-$0.0 and
G338.5+0.1. Detected both by H.E.S.S. and \textit{Fermi}-LAT, we present here a
morphological and spectral analysis of these two sources using 8 years of
\textit{Fermi}-LAT data between 200 \si{\mega\electronvolt} and 1
\si{\tera\electronvolt} with multi-wavelength observations to assess their
nature. The morphology of HESS J1640$-$465 is described by a 2D Gaussian
($\sigma=$ \ang[astroang]{0.053} $\pm$ \ang[astroang]{0.011}$_{stat}$ $ \pm$
\ang[astroang]{0.03}$_{syst}$) and its spectrum is modeled by a power-law with
a spectral index $\Gamma = 1.8\pm0.1_{\rm stat}\pm0.2_{\rm syst}$. HESS
J1641$-$463 is detected as a point-like source and its GeV emission is
described by a logarithmic-parabola spectrum with $\alpha = 2.7 \pm 0.1_ {\rm
stat} \pm 0.2_ {\rm syst} $ and significant curvature of $\beta = 0.11 \pm
0.03_ {\rm stat} \pm 0.05_ {\rm syst} $. Radio and X-ray flux upper limits were
derived. We investigated scenarios to explain their emission, namely the
emission from accelerated particles within the SNRs spatially coincident with
each source, molecular clouds illuminated by cosmic rays from the close-by
SNRs, and a pulsar/PWN origin. Our new \emph{Fermi}-LAT results and the radio
and flux X-ray upper limits pose severe constraints on some of these models.
