This paper investigates the mixing of two supersonic streams in a supersonic exhaust diffuser using theoretical, numerical, and experimental approaches. It has focused attention on the startup and stationary operation of the two-stream supersonic diffusers. The diffuser geometry consists of a constant area throat which connects two ducts of convergent and divergent cross-sections. The effects of diffuser contraction ratio (CR) and convergence angle (θ) on the starting process, pressure recovery, and flow features. Experiments have been performed at five different secondary Mach number (Ms) values in the range of 1.8 < Ms < 2.6, at a fixed primary Mach number (Mp) value of 2. Synchronized pressure measurements are used to study the diffuser performance characteristics. Numerical simulations were also carried out to obtain a better insight into the flow field and computed results have been compared with the experimental results. It was found that the starting behavior has a hysteresis. As a result, the tunnel can be operated at lower pressure ratio than the starting pressure ratio. It is observed that the settling chamber pressure required to start a Constant-Area Diffuser (CAD) is decreased with increase in the diffuser length up to an L/D ratio of 9; subsequently, starting pressure remains almost constant with the further increase in the length due to frictional and pressure recovery losses. The present study provides valuable insights into the multi-stream supersonic mixing and diffusion problem. © 2019 Elsevier Masson SAS