For more on [http://context.aanhet.net/svn/contextman/context-reference/en/co-pagedesign.pdf layout and page design] see the [http://context.aanhet.net/svn/contextman/context-reference/en/co-pagedesign.pdf manual chapter].
At the moment a layoutchange (setuplayout) inside a page is not proper working in mkiv duplex mode.
For more details and actual state see mailing list and [http://wiki.contextgarden.net/User_talk:Zenlima Bug Layoutchange inside page while duplexmode].