OnMouseEnter -/Exit with multiple Cameras

Hey

I got trouble with the Monobehaviour.OnMouseEnter(){} event. I'm using 5 cameras to generate a stereoscopic Image inside a shader which outputs the calculated Image to the original main Camera. But the Methods OnMouseEnter, OnMouseExit and OnMouseOver dont trigger any events. If there is only one Camera in the scene everthing works perfektly, but multiple cameras won't. Is there any connection between cameras and these events? Can I specify a camera which is responsible for that? Or am I just missing something :/?

 void OnMouseEnter() {
     if(!MouseStateHandler.IsCameraControlled) {
             this.gameObject.renderer.material = _highlightMaterial;
     }
     Debug.Log("OnMouseEnter");
 }

This works for 1 camera...

Thanks.