  We report on the discovery and validation of a two-planet system around a
bright (V = 8.85 mag) early G dwarf (1.43 $R_{\odot}$, 1.15 $M_{\odot}$, TOI
2319) using data from NASA's Transiting Exoplanet Survey Satellite (TESS).
Three transit events from two planets were detected by citizen scientists in
the month-long TESS light curve (sector 25), as part of the Planet Hunters TESS
project. Modelling of the transits yields an orbital period of \Pb\ and radius
of $3.41 _{ - 0.12 } ^ { + 0.14 }$ $R_{\oplus}$ for the inner planet, and a
period in the range 19.26-35 days and a radius of $5.83 _{ - 0.14 } ^ { + 0.14
}$ $R_{\oplus}$ for the outer planet, which was only seen to transit once. Each
signal was independently statistically validated, taking into consideration the
TESS light curve as well as the ground-based spectroscopic follow-up
observations. Radial velocities from HARPS-N and EXPRES yield a tentative
detection of planet b, whose mass we estimate to be $11.56 _{ - 6.14 } ^ { +
6.58 }$ $M_{\oplus}$, and allow us to place an upper limit of $27.5$
$M_{\oplus}$ (99 per cent confidence) on the mass of planet c. Due to the
brightness of the host star and the strong likelihood of an extended H/He
atmosphere on both planets, this system offers excellent prospects for
atmospheric characterisation and comparative planetology.
