Could do but...

Controls.xaml

                <ControlTemplate TargetType="TabItem">
                    <Grid x:Name="root"
                          Margin="0,0,15,0">
                    	<ContentControl Content="{TemplateBinding Header}" FontSize="26.667">
                    		<ContentControl.Foreground>
                    			<SolidColorBrush Color="{DynamicResource Gray11}" />
							</ContentControl.Foreground>
                    	</ContentControl>
                    </Grid>
                </ControlTemplate>

MainWindow.xaml

                    <TabItem.Header>
                        <StackPanel Orientation="Horizontal">
                            <TextBlock Text="moo" />
                            <TextBlock Text="cow" />
                        </StackPanel>
                    </TabItem.Header>