  We present a broadband (0.4-70 keV) X-ray spectral analysis of the luminous
infrared galaxy (LIRG) system Mrk 463 observed with Nuclear Spectroscopic
Telescope Array (NuSTAR), Chandra, and XMM-Newton, which contains double active
galactic nuclei (AGNs) (Mrk 463E and Mrk 463W) with a separation of $\sim$ 3.8
kpc. Detecting their transmitted hard X-ray $>10$ keV continua with NuSTAR, we
confirm that Mrk 463E and Mrk 463W have AGNs with intrinsic luminosities of
(1.6-2.2) $\times 10^{43}$ and (0.5-0.6) $\times 10^{43}$ erg s$^{-1}$ (2-10
keV) obscured by hydrogen column densities of 8 $\times 10^{23}$ and 3 $\times
10^{23}$ cm$^{-2}$, respectively. Both nuclei show strong reflection components
from cold matter. The luminosity ratio between X-ray (2-10 keV) and [O IV]
25.89 $\mu$m of Mrk 463E is $\sim$5 times smaller than those of normal Seyfert
galaxies, suggesting that the intrinsic SED is X-ray weak relative to the UV
luminosity. In fact, the bolometric AGN luminosity of Mrk 463E estimated from
$L^{\prime}$-band (3.8 $\mu$m), [O IV] 25.89 $\mu$m, and [Ne V] 14.32 $\mu$m
lines indicate a large bolometric-to-X-ray luminosity ratio, $\kappa_{\rm 2-10
keV} \approx$ 110-410, and a high Eddington ratio, $\lambda_{\rm Edd} \sim$
0.4-0.8. We suggest that the merger triggered a rapid growth of the black hole
in Mrk 463E, which is not yet deeply "buried" by circumnuclear dust. By
contrast, the $L^{\prime}$-band luminosity of Mrk 463W is unusually small
relative to the X-ray luminosity, suggesting that the Eddington ratio is low
($<10^{-3}$) and it might be still in an early phase of merger-driven AGN
activity.
