Composite View Engine
The Composite View Engine just checks whether the view selected to be render is a vm file or aspx files and then delegates the process to the correct view engine instance (AspNetWebForm or NVelocity)
To enable Composite View Engine, configure your web.config viewEngine node as follows:
<viewEngine viewPathRoot="views" customEngine="Castle.MonoRail.Framework.Views.CompositeView.CompositeViewEngine, Castle.MonoRail.Framework.Views.CompositeView" />