  We report the discovery of two TESS sub-Neptunes orbiting the early M dwarf
TOI-904 (TIC 261257684). Both exoplanets, TOI-904 b and c, were initially
observed in TESS sector 12 with twin sizes of 2.49R$_\oplus$ and
2.31R$_\oplus$, respectively. Through observations in five additional sectors
in the TESS primary mission and the first and second extended missions, the
orbital periods of both planets were measured to be 10.887$\pm$0.001 and
83.999$\pm$0.001 days, respectively. Reconnaissance radial velocity
measurements (taken with EULER/CORALIE) and high resolution speckle imaging
with adaptive optics (obtained from SOAR/HRCAM and Gemini South/ZORRO) show no
evidence of an eclipsing binary or a nearby companion, which together with the
low false positive probabilities calculated with the statistical validation
software TRICERATOPS establish the planetary nature of these candidates. The
outer planet, TOI-904 c, is the longest-period M dwarf exoplanet found by TESS,
with an estimated equilibrium temperature of 217K. As the three other validated
planets with comparable host stars and orbital periods were observed by Kepler
around much dimmer stars (J$_{mag}$ $>$ 12), TOI-904 c, orbiting a brighter
star (J$_{mag}$ $=$ 9.6), is the coldest M dwarf planet easily accessible for
atmospheric follow-up. Future mass measurements and transmission spectroscopy
of the similar sized planets in this system could determine whether they are
also similar in density and composition, suggesting a common formation pathway,
or whether they have distinct origins.
