Fusing and slumping are two unique processes, requiring their own unique firing schedules to get right. It’s not possible to conduct these processes simultaneously. A firing schedule designed with the objective of fusing and slumping at the same time will not effectively accomplish either.
Full fusing temperatures are much higher than slumping temperatures. If you were to try to fuse and slump simultaneously by firing to full fuse temperatures, you would end up with a misshapen piece of glass with large air bubbles. The glass would also likely stick to the mold.
Fusing before slumping allows you to avoid potential problems with air bubbles, while also exercising greater control over the final outcome of the piece.
To learn more about firing schedules for fusing and slumping, see TechNote 4: Heat & Glass and TipSheet 7: Platemaking Tips.