The hard X-ray (HXR) imaging spectrometer STIX on board Solar Orbiter gives us the first opportunity to systematically perform stereoscopic X-ray observations of solar flares with a new X-ray fleet: ASO-S/HXI, Aditya-HEL1OS, GECAM, and potentially with PADRE, a CubeSat proposed for this task and carrying STIX cross-calibrated detectors. These observations will provide the first reliable measurements of HXR directivity, our link to the angular distribution of flare-accelerated electrons, and constrain the processes that can accelerate electrons in the corona and in other astrophysical objects. However, such observations must be compared with theory and state-of-the-art modelling taking into account various transport effects and realistic plasma conditions, all of which can change the properties of the measured HXR directivity. Further, measurements of HXR directivity with confidence intrinsically rely on the calibration accuracy of the independently developed detector systems of the different HXR telescopes. In our ISSI team, we propose to bring together experts of each of the different instruments with theorists. The goal of our meetings is to establish requirements on the calibration accuracy derived from simulations so we can distinguish between different theoretical models, and then work out a joint calibration approach of the different detectors to reach the required accuracy. Additionally, the first joint observations between STIX and instruments at Earth (Aditya-HEL1OS and GECAM solar data) will be available in 2021 so it is vital to bring our team together in 2022.