bij verschillende keuzes andere velden

function latenzien(id,veld,waarde)
{
  x = document.form2.elements[veld].selectedIndex;
  if (document.form2.elements[veld].options[x].value > -1)
  {
    document.getElementById(id).style.display = 'block';
  }
  else
  {
    document.getElementById(id).style.display = 'none';
  }
}  

<div id="right_register"><select name="selectieveld" onChange="latenzien('roomname',this.name,'Double')">
      <option value="-1">Single</option>
      <option value="2">Twin (2 separate beds)</option>
      <option value="3">Double</option>
      <option value="4">Triple</option>
    </select></div>
    <div id="roomname" style="display: none">
<div id="left_register">Name :</div>
<div id="right_register"><input type="text" name="roomname1" id="roomname1"  size="50"/></div>
<div id="left_register">Name :</div>
<div id="right_register"><input type="text" name="roomname2" id="roomname2"  size="50"/></div>
</div>