Becaus StringFormat is required and target to is be a string type. You can get around this by either using a TextBlock instead (which has a Text property) or putting the Textblock as the child of the Label:

<Label x:Name="myLabel">
    <Label.Content>
        <TextBlock>
            <TextBlock.Text> <MultiBinding StringFormat="{}{0} - {1}">
                    <Binding Path="Lib1" />
                    <Binding Path="Lib2" />
                 </MultiBinding>
            </TextBlock.Text>
        </TextBlock>
    </Label.Content>
</Label>