Branched endografts have been developed to treat complex pathology in the aortic arch and ascending aorta. This study aims to evaluate the haemodynamic performance of a double-branched thoracic endograft by detailed comparison of flow patterns and wall shear stress in the aorta and supra-aortic branches before and after stent-graft implantation. Pre- and post-intervention CT images were acquired from two patients who underwent thoracic endovascular aortic repair (TEVAR) with a double-branched endograft for thoracic aortic aneurysms. These images were used to reconstruct patient-specific models, which were analysed using computational fluid dynamics employing physiologically realistic boundary conditions. Our results showed that there was sufficient blood perfusion through the arch branches. The presence of inner tunnels caused flow derangement and asymmetric wall shear stress in the ascending aorta, where shear range index was up to 6 times higher than in the pre-intervention model. Wall shear stress in the aortic arch increased considerably after intervention as a result of accelerated flow. The maximum flow-induced displacement forces on the branched endografts were around 22 ​N for both patients, which was below the threshold for device migration. Results from this pilot study demonstrated that aortic flow patterns were significantly altered by the branched endograft which caused increased spatial variation of wall shear stress in the ascending aorta and the arch. Although no obvious adverse hemodynamic features were found immediately after intervention for the cases we analysed, follow-up studies will be needed to assess durability of the device.