I'm a beginner and learning with a video course in Blender Cloud. I would like to apply the same modification to two channels such as the X Scale and Z Scale in one action in the Graph Editor. In the video course, Animation Fundamentals, 101 Simple Bouncing Ball (Part02), around 22:55, the instructor edit both the X scale and Z scale at once.
Use the eye icon to show only the curves you want to edit, then select all keyframes you want to edit, then press G to move, G X to move along time, G Y to move along values, S X to scale along time, S Y to scale along values. Confirm with enter or Left click.
You can set the pivot point of scaling with the usual pivot icon (Bounding box, individual center or 2D cursor).