To reproduce:
1. Have a scheduler grouped by resource 
2. On a button click call its PrintPreview method
3. Open the print settings dialog
4. Select Weekly Style
5. Select Grouped by: Resource
6. Select Two pages per week 
at this point, exception is thrown

Workaround:
  protected override void OnLoad(EventArgs e)
        {
            base.OnLoad(e);
            AddScheduler();

            radScheduler1.PrintSettingsDialogFactory = new MyFactory();
        }

        private void radButton1_Click(object sender, EventArgs e)
        {
            radScheduler1.PrintPreview();
        }

        class MySchedulerPrintSettingsDialogFactory : SchedulerPrintSettingsDialog
        {
            SchedulerPrintStyleSettings userControl;
            RadDropDownList groupBy;
            RadDropDownList twoPagesPerView;
            RadDropDownList printStyle;

            public MySchedulerPrintSettingsDialogFactory(RadPrintDocument document)
                : base(document)  { }

            protected override Control CreateFormatControl()
            {
                userControl = base.CreateFormatControl() as SchedulerPrintStyleSettings;
                RadGroupBox groupBox = userControl.Controls["groupBoxStyleSettings"] as RadGroupBox;

                groupBy = groupBox.Controls["dropDownGroupBy"] as RadDropDownList;
                groupBy.SelectedIndexChanged += groupBy_SelectedIndexChanged;

                twoPagesPerView = groupBox.Controls["dropDownLayout"] as RadDropDownList;
                twoPagesPerView.SelectedIndexChanging += twoPagesPerView_SelectedIndexChanging;

                RadGroupBox groupBoxMain = userControl.Controls["groupBoxPrintStyle"] as RadGroupBox;
                printStyle = groupBoxMain.Controls["dropDownPrintStyle"] as RadDropDownList;
                printStyle.SelectedValueChanged += printStyle_SelectedValueChanged;
                return userControl;
            }

            void printStyle_SelectedValueChanged(object sender, EventArgs e)
            {
                if (printStyle.SelectedItem.Text == "Weekly Style")
                {
                    twoPagesPerView.SelectedItem = twoPagesPerView.Items[0];
                }
            }

            void twoPagesPerView_SelectedIndexChanging(object sender, Telerik.WinControls.UI.Data.PositionChangingCancelEventArgs e)
            {
                if (groupBy.SelectedItem.Text == "Resource" && e.Position == 1)
                {
                    e.Cancel = true;
                    RadMessageBox.Show("Two pages per view is not supported in GroupBy:Resource mode");
                }
            }

            void groupBy_SelectedIndexChanged(object sender, Telerik.WinControls.UI.Data.PositionChangedEventArgs e)
            {
                if (groupBy.Items[e.Position].Text == "Resource")
                {
                    twoPagesPerView.SelectedItem = twoPagesPerView.Items[0];
                }
            }
        }

        class MyFactory : IPrintSettingsDialogFactory
        {
            public Form CreateDialog(RadPrintDocument document)
            {
                return new MySchedulerPrintSettingsDialogFactory(document);
            }
        }