Both Starling and Away3D are rendered using Stage 3D. That's why Starling can be on top of Away 3D. They're at the same depth.
Starling cannot be rendered on top of the classic display list (what we sometimes call the native overlay in Starling), which is always at a higher depth than Stage 3D. Additionally, native controls like StageWebView, or anything added with ANEs, are always at a higher depth than both Stage 3D and the classic display list.