Acceleration is indeed a relative concept in physics, meaning that it depends on the frame of reference from which it is observed. If two objects are traveling at the same velocity relative to each other, they will observe zero acceleration with respect to each other.
To understand this concept, let's consider an example: Suppose you are inside a moving train, and there is another person sitting across from you. From your perspective inside the train, the person sitting opposite you appears to be at rest or stationary. Since you both have the same velocity relative to the train, you observe zero relative velocity between yourselves.
From your frame of reference inside the train, if you were to measure the acceleration of the person opposite you, you would find that their acceleration is zero. This is because they are not changing their velocity relative to you.
Similarly, from the perspective of the person sitting opposite you, they also observe zero acceleration because they perceive themselves to be at rest relative to you. In their frame of reference, you are the one moving at the same velocity, so they see no change in their relative velocity with respect to you.
In both frames of reference, the acceleration is zero because there is no change in velocity between the two objects. This example illustrates the concept of relative acceleration and how it can be perceived differently depending on the chosen frame of reference.