AJAX treats two different GET variables as one?
===

So I'm trying to send two GET variables over AJAX to PHP and then back to the page. For testing purposes I am echoing both variables in my php to see their values. The problem is that both variables have the SAME values, though I know that they actually should have different ones...The structure works like so: there are two dropdown menus. the second one is populated by the php code that is called when user selects an option in the first menu. then, when the user interacts with the second dropdown, the second php code is called by ajax and by now, the only thing it does, is, it echoes those variables. the variables are actually the values from the first and second dropdown menu. 

Please excuse me for possibly a bad description and a bunch of uncommented code. It's late and I'm tired of this issue. Also, I am not experienced with PHP+AJAX, so it will most likely be a very noobish mistake in the script.

This is the html:
   

     <form method="get">
        <select name="firstfilter" onchange="refreshdiv_filterthis(this.value)">
        	<option></option>
        	<option name="optiona">optiona</option>
        	<option name="optionb">optionb</option>
            <option name="optionc">optionc</option>
        </select>
        </form>
        <div id="filterthis"></div>

This is my javascript function:

        function AJAX(){
        try{
        xmlHttp=new XMLHttpRequest();
        return xmlHttp;
        }
        catch (e){
        try{
        xmlHttp=new ActiveXObject("Msxml2.XMLHTTP");
        return xmlHttp;
        }
        catch (e){
        try{
        xmlHttp=new ActiveXObject("Microsoft.XMLHTTP");
        return xmlHttp;
        }
        catch (e){
        alert("Your browser does not support AJAX.");
        return false;
        }
        }
        }
        }
        
        function refreshdiv_filterthis(str){
        
        var divid = "filterthis";
        var url = "getdata.php";
        url=url+"?firstfilter="+str+"&secondfilter="+str;
        url=url+"&sid="+Math.random();
        
        var xmlHttp_one = AJAX();
        
        xmlHttp_one.onreadystatechange=function(){
        if(xmlHttp_one.readyState==4 || xmlHttp_one.readyState=="complete"){
        document.getElementById(divid).innerHTML=xmlHttp_one.responseText;
        }
        }
        xmlHttp_one.open("GET",url,true);
        xmlHttp_one.send(null);
        }
    function refreshdiv_tablehere(str){
    
    var divid = "tablehere";
    var url = "getdata2.php";
    url=url+"?firstfilter="+str+"&secondfilter="+str;
    url=url+"&sid="+Math.random();
    
    var xmlHttp_two = AJAX();
    
    xmlHttp_two.onreadystatechange=function(){
    if(xmlHttp_two.readyState==4 || xmlHttp_two.readyState=="complete"){
    document.getElementById(divid).innerHTML=xmlHttp_two.responseText;
    }
    }
    xmlHttp_two.open("GET",url,true);
    xmlHttp_two.send(null);
    }

and then my getdata.php does something like this:

    <?php
    $host = 'localhost';
    $port = 3306; 
    $database = 'k24868_rt';
    $username = 'k24868_rt';
    $password = 'kdaflnkangak';
    
    $first_filter=$_GET["firstfilter"];
    $second_filter=$_GET["secondfilter"];
   
    $dsn = "mysql:host=$host;port=$port;dbname=$database";
    
    $db = new PDO($dsn, $username, $password);
    
    // First Query!
    $first_sql = "SELECT DISTINCT $first_filter FROM repertoire";
    $filter_q = $db->prepare($first_sql);
    $filter_q->execute();
    
    //Output dropdown list
    	echo "<form method='get'><select name='secondfilter' onchange='refreshdiv_tablehere(this.value)'>";
    	while ( $row = $filter_q->fetchObject() ) {
            echo "<option>";
    		echo "{$row->{$first_filter}}";
    		echo "</option>";
    		}
    	echo "</select></form>";
    
    echo "$first_filter";
    echo "$second_filter";
    
    ?>

That is the getdata2.php that is executed when user selects something in the second dropdown menu:

    <?php
    $host = 'localhost';
    $port = 3306; 
    $database = 'k24868_rt';
    $username = 'k24868_rt';
    $password = 'kmkmalfknasf';
    
    $first_filter=$_GET["firstfilter"];
    $second_filter=$_GET["secondfilter"];
    
    $dsn = "mysql:host=$host;port=$port;dbname=$database";
    
    $db = new PDO($dsn, $username, $password);
   
    //second query
    //$second_sql = "SELECT * from repertoire WHERE $first_filter = $second_filter";
    //$table_output = $db->prepare($second_sql);
    //$table_output->execute();
    	
    //Output
    	    echo "$first_filter";
    echo "$second_filter";
    ?>

Thanks for your help!