I just drew a flow-chart to explain how it should look and how the individual controls should be placed.
In the markup , it would look like:
<telerik:RadSplitter ID="myRadSplitter" runat="server">
<telerik:RadPane ID="navigationPane" runat="server" >
<telerik:RadTreeView ID="TreeView" runat="server" >
<Nodes>
<telerik:RadTreeNode runat="server" Text="1" Target="contentPane">
</telerik:RadTreeNode>
<telerik:RadTreeNode runat="server" Text="2" Target="contentPane">
</telerik:RadTreeNode>
</Nodes>
</telerik:RadTreeView>
</telerik:RadPane>
<telerik:RadSplitBar ID="myRadSplitBar" runat="server" CollapseMode="Forward" />
<telerik:RadPane ID="contentPane" runat="server" >
</telerik:RadPane>
</telerik:RadSplitter>
Hope this helped.