I am not sure I understand your question. You need to add dynamically a user control on the page from an event in another user control. Do you have difficulties with being unable to add the user control, or you have problems with RadTreeView and its server-side event in which you add another user control.
If you cannot add a user control from another user control, try the following. Find your container control that will hold your second control in the Page object:
|(Page.FindControl("SecondControlPanel") as Panel).Controls.Add(mySecondUserControl);
the Telerik team