The 1st function can call 2nd and be simple.

function CompleteMenuItemCaption(const AMenuItem: TMenuItem; Spacing:
String): string;
begin
   Result := AMenuItem.Caption;
   if AMenuItem.ShortCut  scNone then
     Result := Result + Spacing + MenuItemShortCut(AMenuItem);
end;
(* Idem with external string caption *)
function CompleteMenuItemStringCaption(const AMenuItem: TMenuItem;
ACaption: String; Spacing: String): string;
begin
   Result := ACaption;
   if AMenuItem.ShortCut  scNone then
     Result := Result + Spacing + MenuItemShortCut(AMenuItem);
end;