There are a two approaches you can take:
1) when the respective button (OK or Send for example) within the RadWindow is opened you can get a reference to the RadWindow and change its Url so that the next page is loaded inside. This way you will be using only one RadWindow. For more information on the available methods and the way to get a reference to the RadWindow from within its content page please examine the following articles:
First get a reference to the window as described in the first article, then call the function from the main window (accessing it via the BrowserWindow property) that will open the second popup and then in the same button handler call the first window's close() method. The same procedure can be repeated for the second - click the button, get the reference, call the close method. For more information on calling a function from the main page please see the following article: http://www.telerik.com/help/aspnet-ajax/window-programming-calling-functions.html.
On the ways to open a RadWindow please see this page: http://www.telerik.com/help/aspnet-ajax/window-programming-opening.html.
For your convenience I created and attached a small sample that shows these suggestions in action.
I hope my reply was helpful.
the Telerik team