TwoWayView.xaml

<UserControl x:Class="DXBindingExample.View.TwoWayView"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             xmlns:v="clr-namespace:DXBindingExample.View"
             xmlns:vm="clr-namespace:DXBindingExample.ViewModel"
             xmlns:dxmvvm="http://schemas.devexpress.com/winfx/2008/xaml/mvvm"
             xmlns:sys="clr-namespace:System;assembly=mscorlib"
             mc:Ignorable="d" d:DesignHeight="600" d:DesignWidth="800"
             d:DataContext="{dxmvvm:ViewModelSource vm:TwoWayViewModel}">
    <StackPanel Orientation="Vertical">
        <StackPanel Style="{StaticResource groupPanelStyle}">
            <TextBlock Text="Example #1" Style="{StaticResource headerDescriptionStyle}"/>
            <TextBlock Style="{StaticResource descriptionStyle}">
                DXBinding Expr: 'FirstName + ` ` + LastName'.
                <LineBreak/>
                DXBinding BackExpr: 'FirstName = @v.Split(` `[0])[0]; LastName = @v.Split(` `[0])[1];'.
            </TextBlock>
            <StackPanel Orientation="Vertical">
                <TextBlock Text="FirstName: "/>
                <TextBox Text="{DXBinding FirstName, UpdateSourceTrigger=PropertyChanged}" Style="{StaticResource valueStyle}"/>
                <TextBlock Text="LastName: "/>
                <TextBox Text="{DXBinding LastName, UpdateSourceTrigger=PropertyChanged}" Style="{StaticResource valueStyle}"/>
                <TextBlock Text="FullName: "/>
                <TextBox 
                    Text="{DXBinding 
                                Expr='FirstName + ` ` + LastName', 
                                BackExpr='FirstName = @v.Split(` `[0])[0]; LastName = @v.Split(` `[0])[1];',
                                FallbackValue='', Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" 
                    Style="{StaticResource valueStyle}"/>
            </StackPanel>
        </StackPanel>
        <StackPanel Style="{StaticResource groupPanelStyle}">
            <TextBlock Text="Example #2" Style="{StaticResource headerDescriptionStyle}"/>
            <TextBlock Style="{StaticResource descriptionStyle}">
                Function: int Sum(int x, int y) { return x + y; }
                <LineBreak/>
                DXBinding Expr: 'Sum($int.Parse(@e(x).Text), $int.Parse(@e(y).Text))'.
                <LineBreak/>
                DXBinding BackExpr: '@e(x).Text = $int.Parse(@v); @e(y).Text = 0;'.
            </TextBlock>
            <StackPanel Orientation="Vertical">
                <TextBlock Text="X: "/>
                <TextBox x:Name="x1" Text="1" Style="{StaticResource valueStyle}"/>
                <TextBlock Text="Y: "/>
                <TextBox x:Name="y1" Text="2" Style="{StaticResource valueStyle}"/>
                <TextBlock Text="Res: "/>
                <TextBox 
                    Text="{DXBinding 
                                Expr='Sum($int.Parse(@e(x1).Text), $int.Parse(@e(y1).Text))', 
                                BackExpr='@e(x1).Text = $int.Parse(@v); @e(y1).Text = 0;',
                                FallbackValue=0, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" 
                    Style="{StaticResource valueStyle}"/>
            </StackPanel>
        </StackPanel>
    </StackPanel>
</UserControl>