What you are describing that you are trying to do sounds very strange to me. Can you explain with a little more detail your purpose for displaying more than one page in a master template? That really isn't what it was designed to do, even in the core .Net framework.
The way that the mutliple page placeholders on a master template are supposed to work is to give you the ability to split up the page template markup so that you can have something that is essential like this in the master page:
<div id="TopPartOfMasterTemplate">
Content that will be rendered on all pages that use this master template
</div>
<asp:PageContentPlaceHolder id="UnderTopContent" runat="server"/>
<div id="MiddlePartOfMasterTemplate">
Content that will be rendered on all pages that use this master template.
</div>
<asp:PageContentPlaceHolder id="UnderMiddleContent" runat="server"/>
<div id="BottomPartOfMasterTemplate">
Content that will be rendered on all pages that use this master template.
</div>
And on the page that uses that master template, you can have two content controls:
<asp:Content id="Content1" runat="server" ContentPlaceHolderID="UnderTopContent">
This content will appear under the top content
</asp:Content>
<asp:Content id="Content1" runat="server" ContentPlaceHolderID="UnderMiddleContent">
This content will appear under the Middle content
</asp:Content>
This allows you to use a master template to define not only the top and bottom parts of a page, but anything in the middle. It is not, however, intended to be used to display the content of multiple pages.
Could you explain more about what your intentions are and possibly even attach a screenshot for us to see? (use something like imageshack.us to host your image)