<select name="acabados[]">
<option value="1">ABC</option>
<option value="2">DEF</option>
<option value="3">GHI</option>
</select>

<select name="acabados[]">
<option value="1">ABC</option>
<option value="2">DEF</option>
<option value="3">GHI</option>
</select>

$(function () {
    var $select = $('select');
    $select.on('change', function () {
        var output = 0;
        for (var i = 0, len = $select.length; i < len; i++) {
            output += parseInt($select.eq(i).val());
        }
        //the `output` variable now contains the addition of all the values from the `acabados[]` select elements
    });
});

$(document).on('change', 'select[name="acabados[]"]', function(){
    var total = 0;
    $('select[name="acabados[]"]').each(function(){
        total += parseInt($(this).val());
    });
});