[monolithed]

<script type="text/javascript">
window.onload = function(){
    var select = document.forms['select'].elements, i = select.length, array = [];
    while(i--){
        select[i].onchange = function(){
            array.push(this[this.selectedIndex].text);
            select[2].value = array;
        };
    }
};
</script>

<form action="" method="post" name="select">
    <select>
        <option disabled>Выберите1</option>
        <option>1</option>
        <option>2</option>
        <option>3</option>
   </select>
    <select>
        <option disabled>Выберите2</option>
        <option>4</option>
        <option>5</option>
        <option>6</option>
   </select>
   <input type="text" value="" />
   <input type="submit" value="Отправить" />
</form>

Ну и соответственно в элементе input['text'] замените на hidden