What I can suggest you in order to achieve that scenario would be to inherit from the AutoCompleteBox and override the OnKeyDown method. Inside will be able to check whether the Tab key was pressed and move the focus as demonstrated below (TextBox with Name TxtBox will be focused):
CustomAutoCompleteBox : RadAutoCompleteBox
(e.Key == Key.Tab)
// move the focus
var mp = (MainPage)Application.Current.RootVisual;
Hope this helps.
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