答案是这样的：

$pdf=new FPDI();
$pdf->addPage();

// Form
$pdf->setSourceFile('form.pdf'); // Specify which file to import
$tplidx = $pdf->importPage(1); // Import first page
$pdf->useTemplate($tplidx,0,0); // Position at 0,0 and use original page size

// Stamp
$pdf->setSourceFile('stamp.pdf'); // Specify which file to import
$tplidx2 = $pdf->importPage(1); // Import first page
$pdf->useTemplate($tplidx2,10,10,($pdf->w / 2)); // Position at 10,10 and resize to 50% of current page width

$pdf->Output();