Measured versus true mean velocity for co-added two-epoch mock spectra at S/N = 5. Each point represents one realisation. The colour indicates the absolute separation |v₂ − v₁|. The dashed line marks the 1:1 relation. The strong clustering around the 1:1 line shows that full-spectrum fitting of co-added data recovers the period-averaged velocity of the input epochs. Outliers occur only for very large separations, where the stacked line becomes strongly double peaked.