  The Spectrometer/Telescope for Imaging X-rays (STIX) on board Solar Orbiter
(SolO) provides a unique opportunity to systematically perform stereoscopic
X-ray observations of solar flares with current and upcoming X-ray missions at
Earth. These observations will produce the first reliable measurements of hard
X-ray (HXR) directivity in decades, providing a new diagnostic of the
flare-accelerated electron angular distribution and helping to constrain the
processes that accelerate electrons in flares. However, such observations must
be compared to modelling, taking into account electron and X-ray transport
effects and realistic plasma conditions, all of which can change the properties
of the measured HXR directivity. Here, we show how HXR directivity, defined as
the ratio of X-ray spectra at different spacecraft viewing angles, varies with
different electron and flare properties (e.g., electron angular distribution,
highest energy electrons, and magnetic configuration), and how modelling can be
used to extract these typically unknown properties from the data. Lastly, we
present a preliminary HXR directivity analysis of two flares, observed by the
Fermi Gamma-ray Burst Monitor (GBM) and SolO/STIX, demonstrating the
feasibility and challenges associated with such observations, and how HXR
directivity can be extracted by comparison with the modelling presented here.
