$spreadsheet = new Spreadsheet();
$sheet = $spreadsheet->getActiveSheet();
$sheet->setTitle('tab1');

에 setActiveSheetIndex 기능을 이용해서

$spreadsheet->createSheet();
// Add some data
$spreadsheet->setActiveSheetIndex(1) ->setCellValue('A1', 'Hello world!');
// Rename worksheet
$spreadsheet->getActiveSheet()->setTitle('tab2');

를 추가하면 된다.

총 코드

$spreadsheet = new Spreadsheet();
$sheet = $spreadsheet->getActiveSheet();
$sheet->setTitle('tab1');

$spreadsheet->createSheet();
$spreadsheet->setActiveSheetIndex(1) ->setCellValue('A1', 'Hello world!');
$spreadsheet->getActiveSheet()->setTitle('tab2');

$spreadsheet->setActiveSheetIndex(0);