It's impossible. This is not even logical. If you host an independent ContentPlaceHolder, how does the parent main page know where to put it?
How would he build a control tree if the parent was not attached to the controls?
You will need to select the ContentPlaceHolder in the parent to add it to.
If you do this for positioning purposes, you can use CSS to position the contents of this section, regardless of whether you want the page to display.
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