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procedure Memo1Add();
begin
  while true do
    Form1.Memo1.Lines.Append('a');
  ExitThread(0);
end;

procedure Memo2Add();
begin
  while true do
    Form1.Memo2.Lines.Append('b');
  ExitThread(0);
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  CloseHandle(CreateThread(nil, 0, @Memo1Add, nil, 0, dwThreadID));
end;

procedure TForm1.Button2Click(Sender: TObject);
begin
  CloseHandle(CreateThread(nil, 0, @Memo2Add, nil, 0, dwThreadID));
end;

end.