Code

procedure TForm1.FC1ArrowCreated(ASender: TObject;
  const arrow: IArrowItem);
var
  fbox, tbox: IBoxItem;
  Arr2: IArrowItem;
  dx, dy: single;  // vertical vector
  ddx, ddy: integer;  // line direction vector
  nn: single;
  ll: integer;  // parallel interval: vertical vector length
begin
  fbox:=arrow.DestinationBox;
  tbox:=arrow.OriginBox;
  ddx:=arrow.CtrlPtX[1]-arrow.CtrlPtX[0];
  ddy:=arrow.CtrlPtY[1]-arrow.CtrlPtY[0];

  nn:=ll/(sqrt(ddx*ddx+ddy*ddy));
  dx:=ddy*nn;
  dy:=-ddx*nn;
  Arr2:=fc1.CreateArrow(fbox, tbox);
  Arr2.CtrlPtX[0]:=Round(Arr2.CtrlPtX[0]+dx);  // x1
  Arr2.CtrlPtX[1]:=Round(Arr2.CtrlPtX[1]+dx);   // x2
  Arr2.CtrlPtY[0]:=Round(Arr2.CtrlPtY[0]+dy);  // y1
  Arr2.CtrlPtY[1]:=Round(Arr2.CtrlPtY[1]+dy);  // y2
  Arr2.Update;
  Arrow.CtrlPtX[0]:=Round(Arrow.CtrlPtX[0]-dx);
  Arrow.CtrlPtX[1]:=Round(Arrow.CtrlPtX[1]-dx);
  Arrow.CtrlPtY[0]:=Round(Arrow.CtrlPtY[0]-dy);
  Arrow.CtrlPtY[1]:=Round(Arrow.CtrlPtY[1]-dy);
  Arrow.Update;
end;
 

Code

 arrow.PairArrowInterval:=3;
 arrow.DrawPairArrow;