In my game, the player controls a rigidbody2D which to jump / advance makes use of AddForce. But when I add another gameobject with another rigidbody2D, the gravity scale of the first is affected, and it does not respond to any force (in fact, the second sometimes responds to these forces without having any relation to the player script). Any idea of what the hell is hapening to my game?
No idea and they won’t be magically linked as you described. You need to provide more information for anyone to help really.