public class StatisticsDocument
{
    private StatData _statisticsData;
    public StatData StatisticsData
    {
        get { return _statisticsData; }
        set { Set(() => StatData, ref _statisticsData, value); }
    }
}

public class StatData : ObservableObject
{
    private string _sensor1Data;
    public string Sensor1DATA
    {
        get { return _sensor1Data; }
        set { Set(() => Sensor1DATA, ref _sensor1Data, value); }
    }

    private string _sensor2Data;
    public string Sensor2DATA
    {
        get { return _sensor2Data; }
        set { Set(() => Sensor2DATA, ref _sensor2Data, value); }
    }

}

public StatisticsDocument AppendDocumentData(
      StatisticsDocument originaldocument,
      StatisticsDocument importedDocument)
      {
           original.StatisticsData += importedDocument.StatisticsData;
      }