<div id="sample_div">
  <form>
    <div class="form-group field-middle_name row">
      <div class="col-sm-6 col-md-6 col-xs-6 no-padding">
        <select name="prefix_id" id="prefix_id" class="form-control" title="Префикс" required="required">
        <option value="">Префикс</option>
      </select>
      </div>
      <div class="col-sm-6 col-md-6 col-xs-6 no-padding">
        <input type="number" name="sample_number" id="sample_number" class="form-control" placeholder="Номер" title="Номер образца" required="required">
      </div>
    </div>
    <div class="help-block row"></div>
  </form>
</div>


.no-padding{
  padding-right: 0;
  padding-left: 0;
}