Update SELECT multiple list using another SELECT multiple list

[MickShake]

Hi,

I am trying to dynamically update a select list based on choices of a previous list. (with erychynds multiselect plugin)
http://jsfiddle.net/8T6fU/22/

Here is my select example :

<select id="Category" multiple="multiple" onChange="TestSubCat();">
        <option value="FirstCat" id="FirstCat">FirstCat</option>
        <option value="SecondCat" id="SecondCat">SecondCat</option>
</select>

<select id="SubCat" name="SubCat" multiple="multiple">
</select>

What I tried so far :

function TestSubCat()
{
    var select = document.getElementById("SubCat");
    var optG = document.createElement("OPTGROUP");
    var G = select.getElementsByTagName("optgroup");

    //var optGExists = G[0].length < 0;
    //if(typeof G[0].label != null){alert("undef");}
    //alert(G[0].label);
    var optGExists = select.length < 0;

    if(document.getElementById("Category").options[0].selected == true && !optGExists)
    {          
        if(optG.label !== "FirstCat")
        {
            optG.label = "FirstCat";
            select.appendChild(optG);
            optG.appendChild(new Option("reboot","FirstCat[reboot]"));
            optGExists = true;
        }
    }
    if(document.getElementById("Category").options[0].selected == false)
    {
        select.remove(0);
        optGExists = false;
    }

    if(document.getElementById("Category").options[1].selected == true)
    {
        if(optG.label !== "SecondCat")
        {
            optG.label = "SecondCat";
            select.appendChild(optG);
            optG.appendChild(new Option("Port","SecondCat[Port]"));
        }
    }
    if(document.getElementById("Category").options[1].selected == false)
    {
        select.remove(1);
    }

    $("#SubCat").multiselect("refresh");
}

I have a problem with writing the right condition in the if statement to avoid appending optgroup and its options when it's already done.
Also, I hardcoded the index for select.remove() but I don't know what to put to get the index related to the optgroup's options. I've also noticed that with no plugin, when removing, the optgroup label stays. How do I remove it in a clean way ?