Hola
<form>
<select name="padre"
onchange="hijo.disabled = (this.value != '2')">
<option value="1" > valor 1 <option>
<option value="2" > valor 2 <option>
</select>
<select name="hijo"
disabled >
<option value="10" > valor 10 <option>
<option value="20" > valor 20 <option>
</select>
</form>
Saludos
