By typing ObjectA.width = ObjectA.width + ObjectB.width you're giving Object A the width of Object A and Object B together.
Why don't you just give object B width the width of Object B?
ObjectA.width = ObjectB.width
Let's say you want it to change whenever Object B width is being changed, you can do}
Every tick --> set width ( of object B)--> ObjectA.width
Did I get you right?