Multiple xaml pages in Silverlight Project

Question

How can I use multiple xaml file in a single silverlight project.. means I need to change the xaml according to the aspx page..
I have a different aspx pages and each page has a xaml file to it... how can change dynamically into
App.xaml.cs - >  Application_Startup - > this.RootVisual = new MainPage();
 now what I need is to set this dynamically according to the page I loaded.... and any other way to fixed this issue.. and to do this effectively..
 
I m using Silverlight 4, VS2010 with C#....

Thanks..
SUNNY