I found out, AutoComplete is implemented in RadDropDownList more sophisticated than in any other Element.
It got things like
e.AutoCompleteSuggest.SuggestMode = SuggestMode.Contains;
e.AutoCompleteAppend.LimitToList = true;
which is unique. Therefore, it has some advantages over using a TextBoxControl with AutoComplete for the same purpose.
This "Contains" is very useful, f.e. if you have data with "<firstname> <lastname>" and you never know which order the user is typing the words. Regular DropDown with AutoCompleteMode would not support this.
It also works fine, to attach the same DataSource to "DataSource" and "AutoCompleteDataSource", they can coexist.
The user will hardly notice, there are two different popups/dropdowns, depending on what he types.
As a wish for future developments, I would encourage to get this support with AutoCompleteSuggestHelpers and AutoCompleteAppendHelpers in other Elements also.
It's very nice to have control of order of things, since today you often have an heuristic approach, you suggest the item that is most often used, oder most recently used, not just anything that fits in.