Wake Induced Oscillations (WIO) and amplification of Vortex Induced Vibration (VIV) are studied for 2 risers in tandem configuration. The criticality of riser interference is first investigated in term of nondimensional numbers for different types of risers. Theoretical, numerical and experimental approaches are then used simultaneously to define similarity rules and to investigate scale effects with respect to the Reynolds number. Finally, experimental results demonstrating amplification of VIV are given.