Two thumbs, opposite directions.

    With increase/decrease buttons.
    <br />
    <br />
    <telerik:RadSlider ID="RadSlider1" runat="server" SmallChange="5" LargeChange="10"
        ItemType="tick" IsSelectionRangeEnabled="true" Width="400px" Height="60px" SelectionStart="-10"
        SelectionEnd="20" OnClientBeforeValueChange="OnClientBeforeValueChange" OnClientValueChange="OnClientValueChange"
        MaximumValue="50" MinimumValue="-50">
    </telerik:RadSlider>
    <br />
    <br />
    Without increase/decrease buttons. This is more usable.
    <br />
    <br />
    <telerik:RadSlider ID="RadSlider2" runat="server" SmallChange="5" LargeChange="10"
        ItemType="tick" IsSelectionRangeEnabled="true" Width="400px" Height="60px" SelectionStart="-10"
        SelectionEnd="20" OnClientBeforeValueChange="OnClientBeforeValueChange" OnClientValueChange="OnClientValueChange"
        MaximumValue="50" MinimumValue="-50" ShowDecreaseHandle="False" ShowIncreaseHandle="False">
    </telerik:RadSlider>

    <script type="text/javascript">
        var avoidRecusrsion = false;
        function OnClientBeforeValueChange(sender, args) {
            if (avoidRecusrsion) return;
            var newEnd = sender.get_selectionEnd();
            var newStart = sender.get_selectionStart();
            var sliderDragHandles = sender.get_dragHandles();
            var activeDragHandle = sender.get_activeHandle();
 
            if (!activeDragHandle) return;
 
            avoidRecusrsion = true;
            // In case we are moving the selectionStart dragHandle.
            if (activeDragHandle == sliderDragHandles[0]) {
                if (newEnd >= sender.get_maximumValue() && args.get_newValue() <= args.get_oldValue()) {
                    args.set_cancel(true);
                 }
            }
            // In case we are moving the selectionEnd dragHandle.      
            else {
                if (newStart <= sender.get_minimumValue() && args.get_newValue() >= args.get_oldValue()) {
                    args.set_cancel(true);
                }
            }
            avoidRecusrsion = false;
        }
 
        function OnClientValueChange(sender, args) {
            if (avoidRecusrsion) return;
 
            var newValue = args.get_newValue();
            var oldValue = args.get_oldValue();
            var difference = (newValue - oldValue) * -1;
            var sliderDragHandles = sender.get_dragHandles();
            var activeDragHandle = sender.get_activeHandle();
 
            if (!activeDragHandle) return;
 
            avoidRecusrsion = true;
            // In case we are moving the selectionStart dragHandle.                 
            if (activeDragHandle == sliderDragHandles[0]) {
                var rangeEnd = sender.get_selectionEnd() + difference;
                sender.set_selectionEnd(rangeEnd);
            }
            // In case we are moving the selectionEnd dragHandle.      
            else {
                var rangeStart = sender.get_selectionStart() + difference;
                sender.set_selectionStart(rangeStart);
            }
            avoidRecusrsion = false;
        }
    </script>