Михаил

$('#sel1').change(function () {
      areaPrice = this.value;
      const sel2Val = $('#sel2').val()
      total_sum = $('input.total_sum').val(areaPrice + sel2Val  );
    });
 $('#sel2').change(function () {
      areaPrice = this.value;
      const sel1Val = $('#sel1').val()
      total_sum = $('input.total_sum').val(areaPrice + sel1Val);
    });