Send current Item and checkbox value in command parameters

I have a TreeView setup with a HierarchialDataTemplate. It's ItemsSource is bound to a collection of Overlay objects in my viewmodel, where each Overlay has a collection of Layer objects (thus the HierarchialDataTemplate). For each Overlay, I'm displaying a CheckBox and a Label which is simply bound to the Overlay's Name property.

What I'm trying to do is, each time one of the checkboxes is checked/unchecked, the current Overlay and the IsChecked property of the CheckBox will be sent as command parameters to my viewmodel.

If I'm not using the MultiValueConverter, I can send one of the properties fine. But I need to send both as parameters.

Below is the related .xaml for the treeview. I'm only showing the necessary parts and just the Checked trigger because the Unchecked is exactly the same:

<TreeView ItemsSource="{Binding OverlaysViewSource}" Name="LayersTreeView">
    <TreeView.ItemTemplate>
        <HierarchicalDataTemplate ItemsSource="{Binding Layers}" >
            <StackPanel Orientation="Horizontal">
                <CheckBox IsChecked="True">
                    <i:Interaction.Triggers>
                        <i:EventTrigger EventName="Checked">
                                <i:InvokeCommandAction Command="{Binding DataContext.SetVisibilityCmd, RelativeSource={RelativeSource AncestorType=UserControl}}" >
                                    <i:InvokeCommandAction.CommandParameter>
                                        <MultiBinding Converter="{StaticResource multiValueConverter}">
                                            <Binding Path="IsChecked, RelativeSource={RelativeSource AncestorType=CheckBox}" />
                                            <Binding/>
                                        </MultiBinding>
                                    </i:InvokeCommandAction.CommandParameter>
                                </i:InvokeCommandAction>
                            </i:EventTrigger>
                    </i:Interaction.Triggers>
                </CheckBox>
                <Label Content="{Binding Name}" />
            </StackPanel>
        </HierarchicalDataTemplate>
    </TreeView.ItemTemplate>
</TreeView>

So in the MultiBinding, the first one: <Binding Path="IsChecked, RelativeSource={RelativeSource AncestorType=CheckBox}" /> to try and send the checkbox's IsChecked property. However, the value I'm getting in the command is DependencyProperty.UnsetValue.

The second one is just for the current Overlay item, but the whole TreeView is being sent as a parameter.

Update: The Overlay class is a third party control and is used in a lot of places that I can't modify. So I can't just add a property to it.

Update2: I've managed to get the Overlay to send properly. Just need the IsChecked property now.

The binding for IsChecked should use {RelativeSource Self}, since the binding is being applied to the CheckBox via the Style.

Your update to your question shows you've already solved the other one.