Since synced layers are often used to sync together two smaller parts of the same part of a rig, the artwork in layers that are synced together is often intertwined. Hence, it can be useful to edit the artwork in two or more layers that are synced together simultaneously.
Several of the drawing tools in the Tools toolbar have a mode specifically made for that purpose, the Apply to Synced Drawing Layers mode, which can be enabled in the Tool Properties view. When this mode is enabled, your drawing tool will have affect the currently selected layer as well as any layer that is synced to it.
This mode is available for the following tools: The Select tool, the Cutter tool, the Reposition All Drawings tool, the Pencil Editor tool, the Edit Gradient/Texture tool, the Eraser tool and the Paint tool.