One way to achieve this is to hide the Zoombar from the Timelines and to add a separate Zoombar. You can try:

<UserControl x:Class="SilverlightApplication1.MainPage"

    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"

    xmlns:igTimeline="clr-namespace:Infragistics.Silverlight.Timeline;assembly=Infragistics.Silverlight.DataVisualization.Timeline.v9.2"

    xmlns:igControls="clr-namespace:Infragistics.Silverlight.Controls;assembly=Infragistics.Silverlight.DataVisualization.Controls.v9.2"

    Width="700" Height="500">

    <Grid x:Name="LayoutRoot" Background="White">

        <Grid.Resources>

            <Style x:Name="ZoombarStyle" TargetType="igControls:XamWebZoombar">

                <Setter Property="Visibility" Value="Collapsed" />

            </Style>

        </Grid.Resources>

        <Grid.RowDefinitions>

            <RowDefinition />

            <RowDefinition />

            <RowDefinition Height="70" />

        </Grid.RowDefinitions>

        <igTimeline:XamWebTimeline x:Name="Timeline1"

                                   ZoombarStyle="{StaticResource ZoombarStyle}">

        </igTimeline:XamWebTimeline>

        <igTimeline:XamWebTimeline x:Name="Timeline2"

                                   ZoombarStyle="{StaticResource ZoombarStyle}"

                                   Grid.Row="1">

        </igTimeline:XamWebTimeline>

        <igControls:XamWebZoombar x:Name="Zoombar"

                                  Grid.Row="2"

                                  ZoomChanged="Zoombar_ZoomChanged">           

        </igControls:XamWebZoombar>

    </Grid>

</UserControl>

…

private void Zoombar_ZoomChanged(object sender, Infragistics.Silverlight.Controls.ZoomChangedEventArgs e)

{

this.Timeline1.Axis.ScrollPosition = e.NewRange.Scroll;

      this.Timeline1.Axis.ScrollScale = e.NewRange.Scale;

      this.Timeline2.Axis.ScrollPosition = e.NewRange.Scroll;

      this.Timeline2.Axis.ScrollScale = e.NewRange.Scale;

}