Waste acrylonitrile butadiene styrene (ABS) and polycarbonate (PC) as dominant components in waste electrical and electronic equipment (WEEE) plastics show significant potential for recycling, which is severely restricted by efficient separation method. We proposed a novel surface treatment method using sodium hypochlorite for facilitating coseparation of ABS and PC from WEEE plastics by flotation for recycling. Optimization of surface treatment process was performed with response surface methodology using Box-Behnken design. A quadratic model was generated for predicting the floating rate of ABS and PC, and it was also used to optimize the coseparation performance. The optimum conditions were determined and included concentration of 0.05 M, temperature of 69.5 °C, contact time of 56.5 min, and stirring rate of 200 rpm. Under optimum conditions, the coseparation of ABS and PC was effectively achieved; the recovery and the purity of ABS and PC reached 97.4% and 100.0%, respectively. The formation of oxygen-bearing groups and morphological changes were confirmed as major mechanism to induce the surface hydrophilization of ABS and PC. Consequently, this method is feasible for selective coseparation of ABS and PC from WEEE plastics, and it provides technological insights in the sustainable deposal of WEEE plastics.