  HD 156424 (B2 V) is a little-studied magnetic hot star in the Sco OB4
association, previously noted to display both high-frequency radial velocity
(RV) variability and magnetospheric H$\alpha$ emission. We have analysed the
TESS light curve, and find that it is a $\beta$ Cep pulsator with 11 detectable
frequencies, 4 of which are independent $p$-modes. The strongest frequency is
also detectable in RVs from ground-based high-resolution spectroscopy. RVs also
show a long-term variation, suggestive of orbital motion with a period of
$\sim$years; significant differences in the frequencies determined from TESS
and RV datasets are consistent with a light-time effect from orbital motion.
Close examination of the star's spectrum reveals the presence of a
spectroscopic companion, however as its RV is not variable it cannot be
responsible for the orbital motion and we therefore infer that the system is a
hierarchical triple with a so-far undetected third star. Reanalysis of LSD
profiles from ESPaDOnS and HARPSpol spectropolarimetry reveals the surprising
presence of a strong magnetic field in the companion star, with $\langle B_z
\rangle$ about $+1.5$ kG as compared to $\langle B_z \rangle \sim -0.8$ kG for
the primary. HD 156424 is thus the second hot binary with two magnetic stars.
We are unable to identify a rotational period for HD 156424A. The
magnetospheric H$\alpha$ emission appears to originate around HD 156424B. Using
H$\alpha$, as well as other variable spectral lines, we determine a period of
about 0.52 d, making HD 156424B one of the most rapidly rotating magnetic hot
stars.
