DataTrigger.xaml

<Entry Placeholder="First name"
              x:Name="nameEntry"
              Text=""/>

<Entry Placeholder="Last name">
    <Entry.Triggers>
        <DataTrigger TargetType="Entry"
             Binding="{Binding Source={x:Reference nameEntry},
                                               Path=Text.Length}"
             Value="0">
          <Setter Property="IsEnabled" Value="False" />
     </DataTrigger>
   </Entry.Triggers>
</Entry>