The general approach shown in the previous post is about creating a RadWindow on the server. If you need more than once instance you should simply call this method more than one time. I can advise that you examine this article
for more information on creating controls dynamically (short version- the Page Init event). I believe you may also find useful this thread
on opening a RadWindow from the server.
As for performance - the main difference between the NavigateUrl and th ContentTemplate is that the page from the URL is loaded only when the RadWindow is shown while the ContentTemplate is always loaded as a part of the main page, thus slowing the initial page load, yet speeding up the loading of the RadWindow. There shouldn't be much of a performance difference with both approaches, the main points to be considered should be user experience and developer comfort.
You can also examine this article
the Telerik team
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