<form action="b.php" method="post">
	<input type="text" name="ad" />
	<input type="text" name="soyad" />
</form>

<form action="c.php" method="post">
	<input type="text" name="diger" />
	<input type="hidden" name="ad" value="<?php echo $_POST['ad']; ?>" />
	<input type="hidden" name="soyad" value="<?php echo $_POST['soyad']; ?>" />
</form>