Use Stage3DManager in order to merge two different scenes+camera (with same position). Then you can use them as layers when rendering:
var stage3DManager:Stage3DManager;
var stage3DProxy:Stage3DProxy;
//IF FIRST TIME CREATE VIEWS AND LOAD ASSETS
if(stage3DManager == null){
// Define a new Stage3DManager for the Stage3D objects
stage3DManager = Stage3DManager.getInstance(stage);
// Create a new Stage3D proxy to contain the separate views
stage3DProxy = stage3DManager.getFreeStage3DProxy();
stage3DProxy.addEventListener(Stage3DEvent.CONTEXT3D_CREATED, onContextCreated);
stage3DProxy.antiAlias = 2;
stage3DProxy.color = 0x0;
}
mainView= new View3D();
mainView.stage3DProxy = stage3DProxy;
mainView.shareContext = true;
addChild(mainView);
layoutView= new View3D();
layoutView.stage3DProxy = stage3DProxy;
layoutView.shareContext = true;
layoutView.layeredView = true;
addChild(layoutView);
layoutView.scene.addChild(layoutView.camera);
then you only need to render in the right order
mainView.render();
layoutView.camera.transform = mainView.camera.transform;
layoutView.render();