•Flash boiling fuel spray of twin-orifice nozzle is experimentally investigated.•Effects of superheat degree on in-nozzle bubble development are revealed.•The transformation in jet atomization under superheated conditions is studied.•Mechanisms of in-nozzle bubble flow and near-nozzle fuel spray are concluded. To investigate the flash boiling fuel spray characteristics of the twin-orifice nozzle with aviation kerosene, a two-dimensional transparent slit nozzle is designed and studied under a wide range of superheated conditions. Both internal bubble development and near-nozzle jet atomization under subcooled and superheated conditions are studied via a high-speed camera with backlit measurement and image processing methods, accompanied with numerical simulation on in-nozzle two-phase flow for auxiliary analysis. The results indicate the expansion chamber of twin-orifice can increase the time for bubble nucleation and growth and enhance the bubble collision and aggregation. Also, under a higher superheat degree, the bubble volume fraction in the liquid-bubble interaction zone increases, and the bubbles exhausted from discharge-orifice can initiate from the interior to the edge of fuel jet at transitional flash boiling state. Furthermore, the bubble disruption occurs at the jet edge, which greatly enhances the atomization of fuel jet, leading to narrower liquid-core and wider spray distribution of near-nozzle fuel jet. In addition, the mechanisms of in-nozzle bubble development and near-nozzle spray atomization in the twin-orifice nozzle are concluded, which can be elucidated with more fundamental understanding of flash boiling fuel sprays.