Scusa ho fatto un paio di errori:
codice:
...
<select name="sel1" id="sel1" onchange="if (this.selectedIndex.value == '1') {document.getElementById('sel2').style.visibility = 'visible';}">
  <option value="1">opt1</option>
  <option value="2">opt2</option>
  <option value="3">opt3</option>
</select>
<select name="sel2" id="sel2" style="visibility: hidden;">
  <option value="a">optA</option>
  <option value="b">optB</option>
  <option value="c">optC</option>
</select>
Secan