String format using MultiBinding?

you are trying to bind a string to an object. But StringFormat requires its target to be a string type.

try putting a TextBlock in your label content and bind your data to it

<StackPanel>
  <Slider x:Name="sl1" Minimum="10" Maximum="100"/>
  <Slider x:Name="sl2" Minimum="10" Maximum="100"/>
  <Label x:Name="label13" Background="Yellow" Foreground="Black">
    <Label.Content>
        <TextBlock>
          <TextBlock.Text>
            <MultiBinding StringFormat="{}{0} x {1} Test">
              <Binding ElementName="sl1" Path="Value" />
              <Binding ElementName="sl2" Path="Value" />
            </MultiBinding>
          </TextBlock.Text>
        </TextBlock>
    </Label.Content>
  </Label>
</StackPanel>

EDIT
your class Movie must implement the INotificationPropertyChanged interface and your two properties must raise the property changed event with their proprty names!

hope this helps