The concurrent flashover characteristics of two parallel long air gaps are investigated. The gap lengths of the two gaps are the same and are set as 502, 798 or 1200 mm. The wave shape of applied voltage is the short-tail lightning impulse. The voltage between the two gaps and the currents flowing through grounding electrodes are measured using a resistive voltage divider and current transformers, respectively. The development processes of leaders are recorded using the image converter camera IMACON468 (Hadland Corp.).