You're "thinking" wrong. A Masterpage does not "have" child pages. A Child page HAS a master page.
So, to do what you want, you first need a child page composed of Default.aspx and Default2.aspx. So first problem is getting two pages to appear on the same page. Only way I know, is with a base child page that has two iFrames. Each Iframe contains a different DefaultPage. Then the base child page can ALSO HAVE a master page.
Child has master ...
Master does not have child ...
Fred