Multiple Viewports of one scene

### Checklist

- [X] I have searched for [similar issues](https://github.com/isl-org/Open3D/issues).
- [X] For Python issues, I have tested with the [latest development wheel](http://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [X] I have checked the [release documentation](http://www.open3d.org/docs/release/) and the [latest documentation](http://www.open3d.org/docs/latest/) (for `master` branch).

### My Question

Hello guys,
Is it possible to get multiple viewports of one scene?

The code should be looking something like this I think:
self.main_scene = rendering.Open3DScene(self.window.renderer)
self.3d_view_scene.scene = self.main_scene
self.top_view_scene.scene = self.main_scene

But that doesnt work, as the o3d can only be assigned to one SceneWidget.
Is there any other way of achieving the same result? By for example adding multiple cams to one scene and redirecting the cam output to different SceneWidgets?