We report the discovery and characterisation of a super-Earth and a sub-Neptune transiting the bright (K = 8.8), quiet, and nearby (37 pc) M3V dwarf TOI-1266. We validate the planetary nature of TOI-1266 b and c using four sectors of TESS photometry and data from the newly-commissioned 1-m SAINT-EX telescope located in San Pedro Mártir (México). We also include additional groundbased follow-up photometry as well as high-resolution spectroscopy and high-angular imaging observations. The inner,

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... rger planet has a radius of R = 2.37 +0.16 −0.12 R ⊕ and an orbital period of 10.9 days. The outer, smaller planet has a radius of R = 1.56 +0.15 −0.13 R ⊕ on an 18.8-day orbit. The data are found to be consistent with circular, co-planar and stable orbits that are weakly influenced by the 2:1 mean motion resonance. Our TTV analysis of the combined dataset enables model-independent constraints on the masses and eccentricities of the planets. We find planetary masses of M p = 13.5 +11.0 −9.0 M ⊕ (<36.8 M ⊕ at 2-σ) for TOI-1266 b and 2.2 +2.0 −1.5 M ⊕ (<5.7 M ⊕ at 2-σ) for TOI-1266 c. We find small but non-zero orbital eccentricities of 0.09 +0.06 −0.05 (<0.21 at 2-σ) for TOI-1266 b and 0.04 ± 0.03 (<0.10 at 2-σ) for TOI-1266 c. The equilibrium temperatures of both planets are of 413 ± 20 and 344 ± 16 K, respectively, assuming a null Bond albedo and uniform heat redistribution from the day-side to the night-side hemisphere. The host brightness and negligible activity combined with the planetary system architecture and favourable planet-to-star radii ratios makes TOI-1266 an exquisite system for a detailed characterisation.