The following example traps the scrolling messages for a TScrollBox component, keeping the two scroll bars in sync. If either of the scroll bars are scrolled, then the position of the other scroll bar is changed to the same value. The scroll messages are trapped by subclassing the scroll box's WinProc.

Example:
type
{$IFDEF WIN32}
  WParameter = LongInt;
{$ELSE}
  WParameter = Word;
{$ENDIF}
  LParameter = LongInt;
{Declare a variable to hold the window procedure we are replacing}
var
  OldWindowProc : Pointer;
function NewWindowProc(WindowHandle : hWnd;
                       TheMessage   : WParameter;
                       ParamW       : WParameter;
                       ParamL       : LParameter) : LongInt
{$IFDEF WIN32} stdcall; {$ELSE} ; export; {$ENDIF}
var
  TheRangeMin : integer;
  TheRangeMax : integer;
  TheRange : integer;
begin
  if TheMessage = WM_VSCROLL then begin
  {Get the min and max range of the horizontal scroll box}
    GetScrollRange(WindowHandle,
                   SB_HORZ,
                   TheRangeMin,
                   TheRangeMax);
  {Get the vertical scroll box position}
    TheRange := GetScrollPos(WindowHandle,
                             SB_VERT);
  {Make sure we wont exceed the range}
    if TheRange < TheRangeMin then
      TheRange := TheRangeMin else
    if TheRange > TheRangeMax then
      TheRange := TheRangeMax;
  {Set the horizontal scroll bar}
    SetScrollPos(WindowHandle,
                 SB_HORZ,
                 TheRange,
                 true);
  end;
  if TheMessage = WM_HSCROLL then begin
  {Get the min and max range of the horizontal scroll box}
    GetScrollRange(WindowHandle,
                   SB_VERT,
                   TheRangeMin,
                   TheRangeMax);
  {Get the horizontal scroll box position}
    TheRange := GetScrollPos(WindowHandle,
                             SB_HORZ);
  {Make sure we wont exceed the range}
    if TheRange < TheRangeMin then
      TheRange := TheRangeMin else
    if TheRange > TheRangeMax then
      TheRange := TheRangeMax;
  {Set the vertical scroll bar}
    SetScrollPos(WindowHandle,
                 SB_VERT,
                 TheRange,
                 true);
  end;
{ Call the old Window procedure to }
{ allow processing of the message. }
  NewWindowProc := CallWindowProc(OldWindowProc,
                                  WindowHandle,
                                  TheMessage,
                                  ParamW,
                                  ParamL);
end;
procedure TForm .FormCreate(Sender: TObject);
begin
{ Set the new window procedure for the control }
{ and remember the old window procedure.       }
  OldWindowProc := Pointer(SetWindowLong(ScrollBox .Handle,
                                         GWL_WNDPROC,
                                         LongInt(@NewWindowProc)));
end;
procedure TForm .FormDestroy(Sender: TObject);
begin
{ Set the window procedure back }
{ to the old window procedure.  }
  SetWindowLong(ScrollBox .Handle,
                GWL_WNDPROC,
                LongInt(OldWindowProc));
end;