Don't force the active grid to 0

To write several sheets:
$export = new PHPExcel2007Export('Excel Export');
$phpExcel = $export->objPHPExcel;

$sheet = $phpExcel->createSheet()->setTitle('grid1');
$phpExcel->setActiveSheetIndex($phpExcel->getIndex($sheet));
$export->computeData($grid1);

$sheet = $phpExcel->createSheet()->setTitle('grid2');
$phpExcel->setActiveSheetIndex($phpExcel->getIndex($sheet));
$export->computeData($grid2);

$export->getContent();