If you create Object_A with coordinates 100, 100 on a layer with parallax 0, 0 the object will stay at 100, 100. If you move Object_B towards Object_A and B is at 5000, 5000, it will still move towards 100, 100 although Object_A is visible on the screen.
To calculate the Parallax 100, 100 coordinates you must add the viewport to the coordinates of Object_A. But when the camera is moving you change the viewport too and Object_A will move away from Object_B until they reach the edge of the layout (provided 'Unbounded scrolling' is disabled).
I dont know what exactly you want to achieve or how your game works, but I have the feeling that you have to rethink something.