The interaction between two large particles (called tracers) in a dry granular flow made of small particles is studied experimentally and numerically. Depending on the size ratio between tracers and small particles (ranging from 6 to 13) and on the flow thickness (ranging from 6 to 30 small particle diameters) three different regimes are observed. For thin flow thicknesses, tracers get close while flowing and remain in contact. For intermediate thicknesses, tracers flow at a defined distance that increases with the flow thickness, and for large thicknesses, there is an abrupt transition above which tracers move away from each other. The transitional flow thickness increases with tracer size. More surprisingly, for all tracer sizes and all flow thicknesses, tracers get aligned with the flow. These features are explained through a mechanism involving the path of the front tracer which favors tracers regrouping, and the velocity gradient of the flow which tends to repel the tracers. Both are linked through the vertical and longitudinal spacings between tracers which modulate these effects.