Código PHP:
<script type="text/javascript">
function compr(){
if (document.getElementById("capac").checkbox="checked" && document.getElementById("noparte").checkbox=="checked")
{document.getElementById('selbu').disabled = false;}
}
</script>
<select disabled="disabled" id="selbu"><option>a</option></select>
<input type="checkbox" onclick="compr()" id="capac">
<input type="checkbox" onclick="compr()" id="noparte">