The presence of coronary tandem lesions poses a significant challenge for the accurate diagnosis and management of coronary artery diseases. This study set out to provide a deeper understanding of the haemodynamic interactions between tandem obstructive coronary lesions and their impact on different haemodynamic diagnostic parameters. Using a computational fluid dynamic model, validated against in vitro laboratory experiments, we investigated the various combinations of moderate and severe stenoses interchangeably located in the proximal and distal segments of the artery. The investigation was conducted using two diagnostic parameters:​ one hyperaemic-based, i.e., FFR, and one rest-based, i.e., iFR, technique, both of which are commonly used to assess the physiological significance of coronary stenoses. The three main findings of this work are:​ (a) the recovery distance (the immediate local distance affected by the presence of stenosis) is much shorter for the rest-measured diagnostic parameter compared with the hyperaemic-measured diagnostic parameter;​ (b) pressure drop measurements immediately downstream of the stenotic sections overestimate the significance of stenoses, and (c) the presence of a moderate stenosis downstream of a severe stenosis increases FFR value (faster FFR recovery). These findings enhance our understanding of the diagnostic accuracy of hyperaemic-based and rest-based physiological diagnostic coronary assessments and the nuances of using these different techniques when assessing tandem coronary stenoses. This understanding can help inform tailored therapeutic approaches for the management of coronary artery disease.