OK fatto!
Avevo sbagliato il nome dei campi modulo.

PAGINA PRINCIPALE

<html><body>
<form name='form2' method='post' action='register.php' id='form2'>
<input type=text name='campo2' value='' maxlength='60' size='40'>
<input type="button" value="Copia" onclick='{ form2.elements["campo2"].value = centro.form1.elements["campo1"].value; }'>
<iframe width="420" height="100" src="modulo2.htm" id="centro" frameborder="0" scrolling="no"></iframe>
</form></body></html>
PAGINA IFRAME
<html><body>
<form name='form1' method='post' action='register.php'>
<input type=text name='campo1' value='' maxlength='60' size='40'>
<input type="button" value="Copia" onclick='{ parent.form2.elements["campo2"].value = parent.form2.elements["campo2"].value + form1.elements["campo1"].value; }'>
</form></body></html>
COLPEEEEVOLE!