private float offset;

void Start()
{
     offset = MultiScreenPosition();
}

public float MultiScreenPosition()
{
    float offset = 0.0f;
#if !UNITY_EDITOR
    int targetDisplay = _myCamera.targetDisplay;
    for (int i = 0; i < targetDisplay; i++)
    {
        Display display = Display.displays[i];
        if (display != null)
        {
            offset += display.renderingWidth;        
        }
    }
#endif
    return offset;
}

_mousePos = UnityEngine.Input.mousePosition;
_mousePos.x -= offset;
UnityEngine.Vector2 mouse = ProjectPointer(_mousePos.x, _mousePos.y);