Hi Brian,
The property you need is called
BorderSize and you should set it to 0 to remove the borders. A KB article about this property is available below:
http://www.telerik.com/support/kb/aspnet-ajax/splitter/removing-the-border-of-radsplitter-panes-and-the-radsplitter.aspx
Greetings,
Svetlina
the Telerik team