Multiple Transparent Context3D drawing into same Stage3D

In WebGL it's possible to have 2 Contexts drawing into the same Canvas, meaning that you can have a 2D library and a 3D library drawing into the same space without having to know about each other.

I know there are ways to share a Context3D in AIR but it would be nice if the display libraries themselves didn't have to know that other display libraries were being used.