  We report the discovery of two warm sub-Neptunes transiting the bright (G =
9.5 mag) K-dwarf HD 15906 (TOI 461, TIC 4646810). This star was observed by the
Transiting Exoplanet Survey Satellite (TESS) in sectors 4 and 31, revealing two
small transiting planets. The inner planet, HD 15906 b, was detected with an
unambiguous period but the outer planet, HD 15906 c, showed only two transits
separated by $\sim$ 734 days, leading to 36 possible values of its period. We
performed follow-up observations with the CHaracterising ExOPlanet Satellite
(CHEOPS) to confirm the true period of HD 15906 c and improve the radius
precision of the two planets. From TESS, CHEOPS and additional ground-based
photometry, we find that HD 15906 b has a radius of 2.24 $\pm$ 0.08 R$_\oplus$
and a period of 10.924709 $\pm$ 0.000032 days, whilst HD 15906 c has a radius
of 2.93$^{+0.07}_{-0.06}$ R$_\oplus$ and a period of
21.583298$^{+0.000052}_{-0.000055}$ days. Assuming zero bond albedo and full
day-night heat redistribution, the inner and outer planet have equilibrium
temperatures of 668 $\pm$ 13 K and 532 $\pm$ 10 K, respectively. The HD 15906
system has become one of only six multiplanet systems with two warm ($\lesssim$
700 K) sub-Neptune sized planets transiting a bright star (G $\leq$ 10 mag). It
is an excellent target for detailed characterisation studies to constrain the
composition of sub-Neptune planets and test theories of planet formation and
evolution.
