Hi,
Why you want to have textbox inside a textarea?it is not good to use textbox inside textarea..
But you can achieve this by setting CSS styles like below
<asp:TextBox ID="TextBox6" runat="server" TextMode="MultiLine" Height="200px" Width="200px"></asp:TextBox>
<asp:TextBox ID="TextBox7" runat="server" Style="position:absolute;margin:97px 0 0 -180px;padding: 0 0 0 0;"></asp:TextBox>
Thanks,
--RG