Hiermit müsste es gehen:
HTML-Code:
<script language="JavaScript">
<!--
function radiochange() {
  if(document.getElementsByName("radios")[1].checked == true) {
    document.getElementById("text2").style.visibility = "hidden";
  } else {
    document.getElementById("text2").style.visibility = "visible";
  }
}
//-->
</script>
<noscript></noscript>
<form name="Formular">
<input type="radio" name="radios" onClick="radiochange()" onChange="radiochange()" value="1" checked>Radio 1
<input type="radio" name="radios" onClick="radiochange()" onChange="radiochange()" value="2">Radio 2<br>
<input type="Text" name="text1"><br>
<input type="Text" id="text2">
</form>
Ich hoffe das hilft dir weiter

Edit:
Shit, zu spät...