You can add an object to the scene and use the expression to control any of its attributes also. For example, suppose you added a default NURBS or polygonal cylinder named Drum to the scene. You could control the attributes of Ball and Drum with the same expression as in this example:
Ball.scaleY = time/2 + 1;
Drum.scaleX = time + 1;
Drum.scaleY = time + 1;
You can link attributes in different objects—so a change in one attribute alters the behavior of the other. For example, you could cause Drum’s scaleY value to always equal two times Ball’s scaleY value as in this example:
Ball.scaleY = time + 1;
Drum.scaleY = Ball.scaleY * 2;
If you decide to control attributes in two (or more) objects, you can select either object to write the expression. In fact, any object or node in a scene can be selected when you write an expression to control an object other than a particle object.
The advantage of creating separate expressions is that you’ll have two expression names, each presumably named after the object and attribute you’re controlling. Having two expression names makes it easier to find the expression that controls the desired attribute.