This is an unusual use case and a complex question. Most likely you will need to write some code to do what you want to do.
Look at some of the skins that use both a SiteMenu and a PageMenu as these may help you. I don't think you should put 2 SiteMenu controls on the page because they will both show the same menu items. You could add another SiteMapDataSource control to your layout.master and then use mojoTreeView or mojoMenu to create your own menu but you will need to implement your own role filtering similar to what is done in SiteMenu. You will probably want to set a different starting node on the SiteMapDataSource control for the alternate menu. You might find helpful code examples in SiteMenu, PageMenu and ChildPageMenu source code.
Hope it helps,