How to bind control elements to two collections

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C#

XAML

DevExpress

Binding

Hi I am trying to bind my control to two collections (bind the header of a groupbox to a collection and the content to another associated collection).
here is the situation in detail:

I have two tables:

Employees(empID, EmpName, empDepartmentId)

and

Departments(depID, depName)

I want to bind these to a flow layout control (devexpress) so that the control's groupboxes headers show the department Name and the content shows employees in the department

The code below works fine for the header but I can't figure out how to add the employee names.

I put ??? where I should bind to the Employees.

I think I have to bind to a collection that is filtered with the Id of the current Item's depID property but I don't know how, something like this:

XPCollection<employee> DepEmployees = new XPCollection<employee>(new BinaryOperator("empDepartmentId", currentgroupbox.depID????? , BinaryOperatorType.Equal));

and then in Xaml:

<TextBlock Text="{Binding DepEmployees , RelativeSource={RelativeSource AncestorType={x:Type local:MainWindow}}}"/>

What I have tried:

Xaml:

<dxlc:FlowLayoutControl
            x:Name="Departments"
            Background="Transparent"
            ItemSpace="12"
            LayerSpace="12"
            Orientation="Horizontal">
<dxlc:FlowLayoutControl.ItemTemplate>
                <DataTemplate>
                    <dxlc:GroupBox
                        Width="200"
                        Height="250"
                        Content="{Binding}"
                        Cursor="Hand"
                        FontFamily="Segoe UI Light"
                        FontSize="14"
                        Header="{Binding}"
                        MaximizeElementVisibility="Visible"
                        Padding="0">
                        
                        <dxlc:GroupBox.HeaderTemplate>
                            <DataTemplate>
                                <TextBlock Margin="1">
                                    <Run FontFamily="Segoe UI" Text="{Binding depName}" />
                                    <Run Text=" , " />
                                    <Run Text="{Binding depID}" />
                                </TextBlock>
                            </DataTemplate>
                        </dxlc:GroupBox.HeaderTemplate>
                        <dxlc:GroupBox.ContentTemplate>
                            <DataTemplate>
                                
                                    <TextBlock Text="{Binding empName ???????}" />
                                
                            </DataTemplate>
                        </dxlc:GroupBox.ContentTemplate>
                    </dxlc:GroupBox>
                </DataTemplate>
            </dxlc:FlowLayoutControl.ItemTemplate>
        </dxlc:FlowLayoutControl>

CSharp

Departments.ItemsSourcenew = XPCollection(typeof(Departments));