To reproduce: use the following code snippet:

private BindingList<MyEvent> eventsSource = new BindingList<MyEvent>();
private AppointmentMappingInfo appointmentMappingInfo_g = new AppointmentMappingInfo();

public RadForm2()
{
    InitializeComponent();
}

private void RadForm2_Load(object sender, EventArgs e)
{
    radScheduler1.ActiveViewType = radScheduler2.ActiveViewType = SchedulerViewType.Timeline;
    radScheduler1.GetTimelineView().ShowTimescale(Timescales.Days);
    radScheduler2.GetTimelineView().ShowTimescale(Timescales.Days);
    radScheduler1.GetTimelineView().RangeStartDate = radScheduler2.GetTimelineView().RangeStartDate = new DateTime(2015, 9, 1);
    radScheduler1.GetTimelineView().RangeEndDate = radScheduler2.GetTimelineView().RangeEndDate = new DateTime(2015, 9, 30);
    radScheduler1.GetTimelineView().StartDate = radScheduler2.GetTimelineView().StartDate = new DateTime(2015, 9, 1);
    radScheduler1.GetTimelineView().GetScaling().DisplayedCellsCount = radScheduler2.GetTimelineView().GetScaling().DisplayedCellsCount = 7;

    appointmentMappingInfo_g.Start = "Start";
    appointmentMappingInfo_g.End = "End";
    appointmentMappingInfo_g.Summary = "Subject";
    appointmentMappingInfo_g.Description = "Description";
    appointmentMappingInfo_g.Location = "Location";
    appointmentMappingInfo_g.UniqueId = "Id";

    SchedulerBindingDataSource dataSource_l = new SchedulerBindingDataSource();
    dataSource_l.EventProvider.Mapping = appointmentMappingInfo_g;
    dataSource_l.EventProvider.DataSource = eventsSource;
    radScheduler2.DataSource = dataSource_l;

    FillAppointments();
}

private void FillAppointments()
{
    Appointment appointment = new Appointment(new DateTime(2015, 9, 2), new DateTime(2015, 9, 6), "DU 2 AU 6");
    appointment.AllDay = true;

    radScheduler1.Appointments.BeginUpdate();
    radScheduler1.Appointments.Add(appointment);
    radScheduler1.Appointments.EndUpdate();

    MyEvent appointment2 = new MyEvent(new DateTime(2015, 9, 2), new DateTime(2015, 9, 6), "DU 2 AU 6","","","");
    radScheduler2.Appointments.BeginUpdate();
    eventsSource.Add(appointment2);
    radScheduler2.Appointments.EndUpdate();
}

public class MyEvent
{
    public DateTime Start { get; set; }

    public DateTime End { get; set; }

    public string Subject { get; set; }

    public string Description { get; set; }

    public string Location { get; set; }

    public string UniqueId { get; set; }

    public MyEvent(DateTime start, DateTime end, string subject,
        string description, string location, string uniqueId)
    {
        this.Start = start;
        this.End = end;
        this.Subject = subject;
        this.Description = description;
        this.Location = location;
        this.UniqueId = uniqueId;
    }
}

Workaround: specify the end day with 1 second more:
 MyEvent appointment2 = new MyEvent(new DateTime(2015, 9, 2), new DateTime(2015, 9, 6, 0, 0, 1), "DU 2 AU 6", "", "", "");