  Studies of the correlation between different diffuse interstellar bands
(DIBs) are important for exploring their origins. However, the Gaia-RVS
spectral window between 846 and 870 nm contains few DIBs, the strong DIB at 862
nm being the only convincingly confirmed one. Here we attempt to confirm the
existence of a broad DIB around 864.8 nm and estimate its characteristics using
the stacked Gaia-RVS spectra of a large number of stars. We study the
correlations between the two DIBs at 862 nm and 864.8 nm, as well as the
interstellar extinction. We obtained spectra of the interstellar medium
absorption by subtracting the stellar components using templates constructed
from real spectra at high Galactic latitudes with low extinctions. We then
stacked the ISM spectra in Galactic coordinates, pixelized by the HEALPix
scheme, to measure the DIBs. The stacked spectrum is modeled by the profiles of
the two DIBs, Gaussian for $\lambda$862 and Lorentzian for $\lambda$864.8, and
a linear continuum. We obtain 8458 stacked spectra in total, of which 1103
(13%) have reliable fitting results after applying numerous conservative
filters. This work is the first of its kind to fit and measure $\lambda$862 and
$\lambda$864.8 simultaneously in cool-star spectra. We find that the EWs and
CDs of the two DIBs are well correlated with each other. The full width at half
maximum (FWHM) of $\lambda$864.8 is estimated as $1.62 \pm 0.33$ nm which
compares to $0.55 \pm 0.06$ nm for $\lambda$862. We also measure the vacuum
rest-frame wavelength of $\lambda$864.8 to be $\lambda_0 = 864.53 \pm 0.14$ nm,
smaller than previous estimates. We find a solid confirmation of the existence
of the DIB around 864.8 nm based on an exploration of its correlation with
$\lambda$862 and estimation of its FWHM. $\lambda$862 correlates better with
E(BP-RP) than $\lambda$864.8.
