WPF One DataTrigger with multiple Values

Well, as Clemens said, there's nothing wrong with having a converter called multiple times.

But if you really want to do it, here's an approach that works:

<Grid>
    <Grid.Resources>

        <local:ValueByRange x:Key="ValueByRange" />

        <Style x:Key="TextBlockInLabelStyle" TargetType="TextBlock">
            <Setter Property="Background" Value="Transparent"/>
            <Setter Property="Foreground" Value="Black"/>
            <Setter Property="FontSize" Value="12"/>
            <Setter Property="TextAlignment" Value="Center"/>
            <Style.Triggers>
                <DataTrigger Binding="{Binding}" Value="1">
                    <Setter Property="Background" Value="Yellow"/>
                    <Setter Property="Foreground" Value="Black"/>
                    <Setter Property="FontSize" Value="14"/>
                </DataTrigger>
                <DataTrigger Binding="{Binding}" Value="2">
                    <Setter Property="Background" Value="Red"/>
                    <Setter Property="Foreground" Value="White"/>
                    <Setter Property="FontSize" Value="16"/>
                </DataTrigger>
            </Style.Triggers>
        </Style>

    </Grid.Resources>

    <Grid.RowDefinitions>
        <RowDefinition />
        <RowDefinition />
        <RowDefinition />
        <RowDefinition />
    </Grid.RowDefinitions>

    <Slider Grid.Row="0"
            x:Name="Slider1" 
            Interval="1" 
            Minimum="0" 
            Maximum="100" />

    <Slider Grid.Row="1" 
            x:Name="Slider2" 
            Interval="1" 
            Minimum="0" 
            Maximum="100" />

    <Label Grid.Row="2" Content="{Binding ElementName=Slider1, Path=Value, Converter={StaticResource ValueByRange}}">
        <Label.Resources>
            <Style TargetType="{x:Type TextBlock}" BasedOn="{StaticResource TextBlockInLabelStyle}" />
        </Label.Resources>
    </Label>

    <Label Grid.Row="3" Content="{Binding ElementName=Slider2, Path=Value, Converter={StaticResource ValueByRange}}">
        <Label.Resources>
            <Style TargetType="{x:Type TextBlock}" BasedOn="{StaticResource TextBlockInLabelStyle}" />
        </Label.Resources>
    </Label>
</Grid>