Hello Jeff,
You can achieve the desired layout and behavior only by configuring two nested RadSplitters as explained in our online demos below:
http://demos.telerik.com/aspnet-ajax/splitter/examples/overview/defaultcs.aspx
and you do not need to write any code. For your convenience I prepared a sample page which demonstrates the layout you have described. Please examine it and let me know in case I have missed or misunderstood some of your requirements.
Sincerely yours,
Svetlina
the Telerik team