The styles related to multipage are too broad and affect all child iframes, not just the one from multipage.

The second iframe must not inherit any styles

<telerik:RadMultiPage runat="server" SelectedIndex="1">
    <telerik:RadPageView runat="server" ContentUrl="http://www.telerik.com" >
    </telerik:RadPageView>
    <telerik:RadPageView runat="server">
        <div>
            <iframe src="http://www.telerik.com" ></iframe>
        </div>
    </telerik:RadPageView>
</telerik:RadMultiPage>