Hi,
Base on my understanding, you want to add more than one contentplachoder, then make it like tabs to load more than one content page, right?
I think there is some misunderstanding about the master page, i donot think you do like your imagine.
In fact, the master page is entirely different from the iframe, the master page class derives from the usercontrol class, when the application is executing, the master page just like a child control.
And, the ContentPlaceHolder of the master which defines a region of the master page rendering that can be substituted with content from a page associated to the master, and you will found the URL that on the Browser Address bar is the content page;s url, not the master page's.
I would suggest a good article, it talks how the master page works: http://www.odetocode.com/Articles/450.aspx
Hope it you!
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Amanda Wang
Microsoft Online Community Support