<script src="https://code.jquery.com/jquery-latest.min.js"></script>
<script>
$(document).ready(InitPage);
function InitPage() {
   var inputs={}, 
        actionResult = "",
        action = $('#testform').attr('action');

   $('#testform select').each(function(){
      getQuery(this);
   });
   $('#testform select').on('change',function(){
      getQuery(this);
      if(Object.keys(inputs).length == 2) result.innerText = action + actionResult;
   });
   function getQuery(el){
       if($(el).val()!=''){
           inputs[$(el).attr('name')]=$(el).val();
           actionResult =  $.param(inputs)?'?'+$.param(inputs):'';
        }else{
           delete inputs[$(el).attr('name')];
           actionResult = '';
           result.innerText = '';
       }
   }
}
</script>
<form id="testform" action="get.php" method="GET">
    <select name="sel1" >
        <option value="">Выберите</option>
        <option value="1">Пиво</option>
        <option value="2">Спирт</option>
        <option value="3">Лимонад</option>
    </select>
    <select name="sel2" >
        <option value="">Выберите</option>
        <option selected value="3">Колбаска</option>
        <option value="4">Мясо</option>
		<option value="5">Деньги</option>
    </select>  
</form>
<div id="result"></div>