Please note that this problem is not directly related to the RadControls but is a general ASP.NET knowledge. When using a MasterPage the scope differs from the standard web page. In order to get a reference to an item from within the content place holder you need to access it as a child to that content place holder, e.g.:
alert($find('<%= this.ContentPlaceHolder1.FindControl("RadSplitter1").ClientID %>').get_id());
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