Composite View Engine
The Composite View Engine just checks whether the view selecteded to be rendered is a .vm file or .aspx file. It then delegates the process to the correct view engine instance: AspNetWebForm or NVelocity.
To enable the 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" />