You are checking the Length of the Mid Point vector, which basically means you are checking the distance betwean the (0,0,0) point and the mid point, so the camera pans out as long as the mid point is 900 units away from the center of the world.
What you need is (probably) the distance betwean the two players.
There is a node for that, just take a pin from one player and type “distance”, you’ll see it.
By the way, you don’t have to split and recombine vectors like that, you can add them and divide them as they are.