You need to put a content placeholder in the nested master page
So effectively, you have a content placeholder in master page say "MainContent" then in your nested master page instead of
<asp:content id="Content2" contentplaceholderid="MainContent" runat="server" xmlns:asp="#unknown"></asp:content>
use
<asp:content id="Content2" contentplaceholderid="MainContent" runat="server" xmlns:asp="#unknown">
<asp:contentplaceholder id="MainContentNested" runat="server">
</asp:contentplaceholder>
</asp:content>
Now in your page refer to MainContentNested instead of MainContent.
Hope that helps