This study presents the first systematic experimental investigation of the streamwise evolution of multi-orifice synthetic jets (MOSJs) driven by a single actuator. A centred hexagonal arrangement of seven circular orifices is examined using particle image velocimetry. Four cases are studied in which Reynolds number and Strouhal number are held constant while the orifice diameter is varied, thereby isolating the effect of the internal orifice geometry through changes in throat aspect ratio. The flow development is shown to consist of four distinct regimes: (i) a near-orifice region dominated by the balance of blowing and suction; (ii) development of individual jets; (iii) a jet-merging regime; and (iv) a merged-jet region that evolves towards self-similar scaling. Despite matched non-dimensional actuation parameters, differences in momentum flux, Reynolds number and turbulence statistics are observed. These differences scale monotonically with orifice diameter and are traced to changes in the exit velocity profile imposed by the orifice throat length. In particular, more diffuse shear layers – resulting from a longer orifice throat – lead to more efficient jet merging, higher downstream momentum and reduced dissipation. The results demonstrate that actuator-internal geometry plays a critical role in MOSJ performance and cannot be neglected in predictive or design-oriented models.