How can I use two binding for <textbox> in WPF with MVVM?

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WPF

MVVM

I'm writing adding,updating and deleting operations for <ListView> in WPF using by MVVM.
My question is that How can I give two binding for the same <TextBox>(one of these is for adding element to <ListView>and the other is for updating(selectedItem of <ListView>))

What I have tried:

When I write binding for adding it works successfully.And also When I write binding for updating the selected row is written to <TextBox>.But I need these work together.I tried <MultiBinding> but result weren't be successfull.Hope your helps..

For adding:

<TextBox Grid.Column="1" x:Name="txtSurname" Text="{Binding Path=Student.Surname, Mode=TwoWay}">
    <TextBox.Effect>
        <DropShadowEffect  ShadowDepth="3" Color="#75B07A"/>
    </TextBox.Effect>
</TextBox>-->

For updating:

<TextBox Grid.Column="1" x:Name="txtSurname" Text="{Binding ElementName=lstStuds, Path=SelectedItem.Surname}">
    <TextBox.Effect>
        <DropShadowEffect  ShadowDepth="3" Color="#75B07A"/>
    </TextBox.Effect>
</TextBox>

This is my <listview> :

<ListView x:Name="lstStuds" ItemsSource="{Binding Path=Students}" SelectedItem="{Binding SelectedStudent}"  Grid.Column="1" Grid.Row="1" Grid.RowSpan="3" Margin="0,10,20,20" FontSize="16">
           <ListView.Effect>
               <DropShadowEffect  ShadowDepth="3" Color="#75B07A"/>
           </ListView.Effect>
           <ListView.View>
               <GridView>
                   <GridViewColumn Header="Name" DisplayMemberBinding="{Binding Name}" Width="150"/>
                   <GridViewColumn Header="Surname" DisplayMemberBinding="{Binding Surname}" Width="150"/>
               </GridView>
           </ListView.View>

And my buttons:

 <Button Content="Add" Command="{Binding AddCommand}" Width="120" Height="auto" FontSize="18" Margin="0 0 0 10">
                    <Button.Effect>
                        <DropShadowEffect ShadowDepth="3" Color="#75B07A"/>
                    </Button.Effect>
                </Button>

<Button Content="Update" Command="{Binding EditCommand}" CommandParameter="{Binding ElementName=lstStuds, Path=SelectedItem}" Width="120" Height="auto" FontSize="18" Margin="0 0 0 10">
                  <Button.Effect>
                        <DropShadowEffect ShadowDepth="3" Color="#75B07A"/>
                    </Button.Effect>
                </Button>