The pairing of helical tip vortices in the wake of a two-bladed rotor is investigated experimentally. Time-resolved particle image velocimetry measurements provide a clear temporal and spatial evolution of the vortical structures, highlighting the transition to instability and the effect of tip speed ratio and helical spacing. The temporal growth rate of the vortex system instabilities were measured and are shown to be dependent on helical spacing. The evolution of filaments and their growth rates support the argument that the mutual inductance mode is the driving mechanism behind the transition to an unstable wake. The measurements are in agreement with maximum growth rates predicted by linear stability analyses of single- and double-helix arrangements. In addition, the wake topology due to varying rotor load through tip speed ratio variation is shown to play an important role in the initial symmetry breaking that drives the wake transition.