  Hot jupiters (P < 10 d, M > 60 $\mathrm{M}_\oplus$) are almost always found
alone around their stars, but four out of hundreds known have inner companion
planets. These rare companions allow us to constrain the hot jupiter's
formation history by ruling out high-eccentricity tidal migration. Less is
known about inner companions to hot Saturn-mass planets. We report here the
discovery of the TOI-2000 system, which features a hot Saturn-mass planet with
a smaller inner companion. The mini-neptune TOI-2000 b ($2.70 \pm 0.15
\,\mathrm{R}_\oplus$, $11.0 \pm 2.4 \,\mathrm{M}_\oplus$) is in a 3.10-day
orbit, and the hot saturn TOI-2000 c ($8.14^{+0.31}_{-0.30}
\,\mathrm{R}_\oplus$, $81.7^{+4.7}_{-4.6} \,\mathrm{M}_\oplus$) is in a
9.13-day orbit. Both planets transit their host star TOI-2000 (TIC 371188886, V
= 10.98, TESS magnitude = 10.36), a metal-rich ([Fe/H] =
$0.439^{+0.041}_{-0.043}$) G dwarf 174 pc away. TESS observed the two planets
in sectors 9-11 and 36-38, and we followed up with ground-based photometry,
spectroscopy, and speckle imaging. Radial velocities from CHIRON, FEROS, and
HARPS allowed us to confirm both planets by direct mass measurement. In
addition, we demonstrate constraining planetary and stellar parameters with
MIST stellar evolutionary tracks through Hamiltonian Monte Carlo under the PyMC
framework, achieving higher sampling efficiency and shorter run time compared
to traditional Markov chain Monte Carlo. Having the brightest host star in the
V band among similar systems, TOI-2000 b and c are superb candidates for
atmospheric characterization by the JWST, which can potentially distinguish
whether they formed together or TOI-2000 c swept along material during
migration to form TOI-2000 b.
