Sewage sludge poses a major challenge for resource utilization and thermal disposal because of its high moisture content, low calorific value, and the slagging and corrosion risks associated with alkali and alkaline earth metals (AAEMs). Synergistic incineration with organic-rich fermentation residue offers a potential route for improving sludge thermal treatment; however, the particle-scale evolution and in-flame transformation behavior of AAEMs during this process remain insufficiently understood. In this study, the synergistic incineration of sludge and fermentation residue was investigated using phase-selective laser-induced breakdown spectroscopy (PS-LIBS) and OH planar laser-induced fluorescence (OH-PLIF), combined with offline X-ray diffraction and fluorescence measurements. PS-LIBS was used to track the relative spatial evolution of particle-phase Na, K, Ca, and Al, while OH-PLIF provided information on flame structure and reaction-zone distribution. The results suggest a possible particle-evolution pathway involving volatile release, fragmentation, alkali vaporization, downstream re-capture, and agglomeration, ultimately leading to the formation of composite inorganic phases. The roles of AAEMs, S, and Cl were further discussed to interpret alkali redistribution and particle-phase signal evolution. Finally, the NO_x emissions during the co-combustion of sludge and fermentation residues are examined which showed a certain degree of reduction. These findings provide particle-scale insight into AAEM transformation during sludge–fermentation residue synergistic incineration and offer a useful reference for clean thermal treatment of heterogeneous organic wastes.