In an FCC process and system, a whole crude oil feedstock is directly converted into light olefins and other products. The feed is separated into a high boiling fraction and a low boiling fraction, and each fraction is processed in separate FCC downflow reactors. The catalyst, combined from both downflow reactors, is regenerated in a common vessel. The low carbon content in the catalyst particles from the low boiling fraction downflow reactor is insufficient to provide the necessary heat. By combining catalyst particles from the high boiling fraction having high carbon content helps to provide additional heat for regeneration.