Lightning is a magnificent natural experiment of dielectric breakdown on a vast scale, providing us rare opportunities to explore this powerful and dangerous phenomenon hidden deeply in the thunderclouds. Specifically, how lightning is initiated in the insulating air is still poorly understood. Using the radio-imaging technique, scientists remotely sensed the very-high frequency (VHF) radiation of lightning and found that a system of positive polarity streamers, named fast positive breakdown (FPB), could be the initiating discharges of some lightning. Streamers are potential plasma waves composed of ionized air molecules, which can gradually transition the insulating air into hot, high conductivity lightning plasma channels. Recently, lighting has also been reported to be initiated by a burst of negative polarity streamers, called fast negative breakdown (FNB). However, the laboratory experiments and simulations of dielectric breakdown suggest that FPB should be more readily produced compared with FNB. Here, we use our high bandwidth (>200 MHz) and fast time resolution (<0.5 microseconds) VHF radio-imaging system, reporting the first observational evidence of FPB preceding FNB and of simultaneous development of positive and negative polarity streamers in lightning initiation. These observations represent a significant addition to our current physical understanding of natural dielectric breakdown in thunderstorms.