Hola:
El atributo que debes consultar es chrcked:
<select disabled="disabled" id="selbu" name="a" >
<option>a</option>
</select>
<input type="checkbox" onclick="a.disabled = (this.checked && this.nextSibling.checked)" id="capac"><input type="checkbox" onclick="a.disabled = (this.checked && this.previousSibling.checked)" id="noparte">
Saludos
