Use the OnDblClick event. This, for example, is from my "player".
procedure TMainForm.ListDblClick(Sender: TObject);
{ Double clicking a filename in the list plays it inmediately }
begin
if FileList.ItemIndex >= 0 then
PlaySong(FileList.ItemIndex);
end;
FileList is a TListBox and this is its OnDblClick handler. We first check whether there's any element selected and if so, call one of two overloads which accepts a "song" index and calls the other overload with the corresponding item string (a file name):
procedure TMainForm.PlaySong(const Index: Integer);
{ Plays the file whose index in the list is Index }
begin
if (Index >= 0) and (Index < FileList.Count) then begin
Current := Index;
PlaySong(FileList.Items[Index]);
end;
end;
In your case you'd do something like:
SecondList.Items.Add(FirstList.Items[FileList.ItemIndex])
HTH!