You can do it with the creation of one custom class, so that is one hoop, but the hoop doesn't know about anything in your Xaml...

public class DoubleCache : INotifyPropertyChanged {
    public event PropertyChangedEventHandler PropertyChanged;
    private double _Value;
    public double Value {
        get { return _Value; }
        set {
            _Value = value;
            if (PropertyChanged != null)
                PropertyChanged(this, new PropertyChangedEventArgs("Value"));
            }
        }
    }

Then you can set up the actual bindings in Xaml...

<UserControl x:Class="TwoSliders.Page"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:local="clr-namespace:TwoSliders;assembly=TwoSliders"
             Width="400" Height="300">
    <StackPanel x:Name="LayoutRoot" Background="White">
        <StackPanel.Resources>
            <local:DoubleCache x:Key="SliderPosition"/>
        </StackPanel.Resources>
        <Slider Value="{Binding Value, Mode=TwoWay, Source={StaticResource SliderPosition}}"/>
        <Slider Value="{Binding Value, Mode=TwoWay, Source={StaticResource SliderPosition}}"/>
    </StackPanel>
</UserControl>