As far as I understand you are using a RadWindow with NavigateUrl instead of a ContentTemplate as Shinu thought. If so, to achieve waht you want you should go through the following steps:
1) Use the Response.Redirect method in the server handler of the button of Page1.aspx.
2) Put the following script in Page2.aspx:
(window != window.top) window.top.location.href = window.location.href;
The step in 1) will cause the Page2.aspx to start loading and the code from 2) will force it to load in the main browser window instead of the RadWindow.
Please, test my suggestion and let me know how it goes.
All the best,
the Telerik team
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