If you want to bind the same view model on multiple views, you need to register the name for each view, differentiating it from the other views. Later, the name will be used as the navigation target. See the following code.

// For iOS
[RegisterNavigation("FirstTab")]
public partial class FirstViewController : UIViewController<SharedViewModel>
{
}

// For Android
[RegisterNavigation("FirstTab")]
public class FirstFragment : Fragment<SharedViewModel>
{

}

// For WinPhone
[ViewModelType(typeof(SharedViewModel))]
[RegisterNavigation("FirstTab")]
public partial class FirstTab : PhoneApplicationPage
{

}

// For WinRT
[ViewModelType(typeof(SharedViewModel))]
[RegisterNavigation("FirstTab")]
public sealed partial class FirstTab : LayoutAwarePage
{

}

Then while initializing the tab items on your parent's view model, you need to specify the navigation target, which contains the child's view model type and the target name. See the following code.

public class TabViewModel : MultiPageViewModelBase
{
    public TabViewModel()
    {
        var items = new List<NavigationItem>();
        items.Add(new NavigationItem("First Tab", new NavigationTarget(typeof(SharedViewModel), "FirstTab")));
        items.Add(new NavigationItem("Second Tab", new NavigationTarget(typeof(SharedViewModel), "SecondTab")));
        items.Add(new NavigationItem("Third Tab", new NavigationTarget(typeof(SharedViewModel), "ThirdTab")));
        this.Items = items.ToArray();
    }
}

Hope this helps. I also attached a sample solution for this scenario.

Feel free to discuss it further if you have any other thought.

Kind Regards,

Jack