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// Zwei Methoden müssen vereinbar werden
type
  TForm1 = class(TForm)
    {...}
    procedure TreeView1Proc(var message:TMessage);
    procedure TreeView2Proc(var message:TMessage);
    procedure FormCreate(Sender: TObject);
    {...}
  end;

implementation

// Zwei "Variablen" vereinbart:

var OldTreeView1Proc:TWndMethod;
    OldTreeView2Proc:TWndMethod;

procedure TForm1.TreeView1Proc(var message:TMessage);
begin
 case message.Msg of
  WM_HSCROLL,WM_VSCROLL:
   OldTreeView2Proc(message);
 end;
 OldTreeView1Proc(message);
end;

procedure TForm1.TreeView2Proc(var message:TMessage);
begin
 case message.Msg of
  WM_HSCROLL,WM_VSCROLL:
   OldTreeView1Proc(message);
 end;
 OldTreeView2Proc(message);
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
 OldTreeView1Proc:=TreeView1.WindowProc;
 OldTreeView2Proc:=TreeView2.WindowProc;
 TreeView1.WindowProc:=TreeView1Proc;
 TreeView2.WindowProc:=TreeView2Proc;
end;