TabControl.ItemTemplate: set TabItem.Header.Text to a MultiBinding with StringFormat

The TabControl contains a ContentTemplate property as well as the ItemTemplate that it inherits from ItemsControl. It uses the ContentTemplate to differentiate what is showing in the Content area while the ItemTemplate which defines the template for the Header. Additionally, each Item from your ItemSource will automatically be wrapped in a TabItem; it doesn't need to be re-created in the ItemTemplate, as that will attempt to place a TabItem inside the Header as you are noticing.

Instead of re-creating a TabItem inside the ItemTemplate, use the ItemTemplate to define your Header content, and the ContentTemplate to define your Content.

<TabControl ItemsSource="{Binding}">    <TabControl.ItemTemplate>        <DataTemplate>            <TextBlock>                <TextBlock.Text>                    <MultiBinding StringFormat="{}{0}--{1}">                        <Binding Path="Title" />                        <Binding Path="Category.Title" />                    </MultiBinding>                </TextBlock.Text>            </TextBlock>        </DataTemplate>    </TabControl.ItemTemplate>    <TabControl.ContentTemplate>        <DataTemplate>            <TextBlock Text="{Binding MyContent}" />        </DataTemplate>    </TabControl.ContentTemplate></TabControl>

In your first paragraph you mentioned wanting to set different sizes on the bound portions of the Header. If you do want to do that, you won't be able to use a single Binding or MultiBinding to set the Text as is done above. Instead you can nest TextBlocks to achieve this with different formatting for each.

<TabControl.ItemTemplate>    <DataTemplate>        <TextBlock>            <TextBlock Text="{Binding Title}"                       FontSize="12" />            <Run Text="--" />            <TextBlock Text="{Binding Category.Title}"                       FontSize="10" />        </TextBlock>    </DataTemplate></TabControl.ItemTemplate>