JSFiddle - React, Tailwind, and code Playground

HTML

<body>
    <p>Prima select</p>
    <select class="sel">
        <option value="1" >Val1</option>
        <option value="2" >Val2</option>
        <option value="3" >Val3</option>
    </select>
    
    <p>Seconda Select</p>
    <select class="sel">
        <option value="4" >Val4</option>
        <option value="5" >Val5</option>
        <option value="6" >Val6</option>
    </select>
    
    <p>Totale: <span id="tot">0</span> </p>
</body>

JavaScript

$( function(){
    $(".sel").prop("selectedIndex",-1);
});

$(".sel").change( function() {
    var tot = 0;
    $(".sel").each( function() {
        var tmp = parseInt($(this).val());
        if (!isNaN(tmp))
            tot += tmp;
      });
    $("#tot").text(tot);
});