Having scene graphs in multiple places is on a long term wish list, but
we don't have a good handle on how to implement it and maintain a lot of
our optimizations. Since nodes cache information that can only be
determined by scanning their ancestors we'd have to have a complicated
multiple-ancestry registry of this information or we'd have to have the
trees disable their optimizations when they appear in more than one place.
In short, we're aware of the feature and still working out how to best
integrate it into our implementation...