I have windows phone solution ; which contain two c# projects. I have a button in one in the pages in the first project ; which we redirect the user to another page in the secon
If you are referencing the other project, you can use the following method:
NavigationService.Navigate(new Uri("/YourOtherProjectName;component/TargetPage.xaml", UriKind.Relative));
EDIT: If anyone is interested, I've put a sample project on Github: https://github.com/tpetrina/blogcode/tree/master/BlogResearch. WP7 application project is called NavigateToLibraryPage and it contains a single button that navigates to the page in the referenced class library named wp7.library