Hola proba el código de abajo.
Código HTML:
<html>
<head>
<script>
function checkRaddio(id){
var x = 0;
var value = null;
while(x<id.length){
if(id[x].checked)value = id[x].value;
x++;
}
return value == null ? "No hay seleccion" : value;
}
function url(){
document.forms.form2.action = checkRaddio(document.forms.form2);
}
</script>
</head>
<body>
<form name="form2" id="form2" onsubmit="url();" action="" method="posT">
<input type="radio" id="radio" name="radio" value='pagina.php'>
<input type="radio" id="radio" name="radio" value='otrapagina.php'>
<input type="submit" value="Siguiente">
</form>
</body>
</html>