<script type="text/javascript">
  var readonly = false;
</script>

<form action="#">
  A: <input type="text" onchange="if (this.value == 'single') readonly = true;" />
  B: <input type="text" onfocus="if (readonly == true) this.blur();" />
</form>

<script language="JavaScript" type="text/javascript">
function changeLists(){
	// In die Stadt
	if(document.getElementById("stand").value == "single"){
		document.getElementById("stand_02").disabled = 1;
	}
	else{
		document.getElementById("stand_02").disabled = 0;
	}
}
</script>
<select id="stand" name="stand" size="1" onChange="changeLists();">
    <option selected>-</option>
    <option value="single">Single</option>
    <option value="married">Verheiratet</option>
    <option value="divorced">Geschieden</option>
</select>
<select id="stand_02" name="stand_02" size="1">
    <option selected></option>
    <option>Single</option>
    <option>Verheiratet</option>
    <option>Geschieden</option>
</select>