Prueba este codigo:
Código PHP:
<html>
<head>
<script type="text/javascript">
window.onload=function(){
//Deshabilitar los selects al cargar el DOM
document.getElementById('select1').disabled=true;
document.getElementById('select2').disabled=true;
}
function seleccionar(selectEnabled,selectDisabled){
document.getElementById(selectEnabled).disabled=false;
document.getElementById(selectDisabled).disabled=true;
}
document.getElementsByName('opcion').onclick=function(){
}
</script>
</head>
<body>
<br>Selecciona:<br>
<input type="radio" name="opcion" value="1" onclick="seleccionar('select1','select2')"/>
<input type="radio" name="opcion" value="2" onclick="seleccionar('select2','select1')"/>
<br>Combos<br>
<select id="select1">
<option>Ocpion 1</option>
<option>Ocpion 2</option>
</select>
<br><br>
<select id="select2">
<option>Ocpion 1</option>
<option>Ocpion 2</option>
</select>
</body>
</html>
Salu2