How to concatenate strings in Xamarin.Forms?

Use FormattedText property in xamarin forms Label as follow:

<Label Grid.Column="1" Grid.Row="1">
    <Label.FormattedText>
        <FormattedString>
            <Span TextColor="White" FontSize="18" Text="{Binding EMP_LAST_NAME'}"/>
            <Span TextColor="White" FontSize="18" Text="{Binding EMP_FIRST_NAME}"/>
        </FormattedString>
    </Label.FormattedText>
</Label>

Xamarin.Forms Label

And also you can add Style to avoid code duplication for TextColor, FontSize and other properties in your code.

Styling Xamarin.Forms Apps using XAML Styles

You can't bind to multiple properties on a View Element.

In this case you should create a new property which does the format you want and bind it to the View.

Example:

public class EmployeeViewModel
{
    public string FirstName { get; set; }    
    public string LastName { get; set; }    
    public string FullName => $"{FirstName} {LastName}";
}

Then in XAML:

<Label Text="{Binding FullName}"/>

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Another approach:

As suggested in the comments we can also use FormattedText property in a Label:

<Label.FormattedText>
   <FormattedString>
     <Span Text="{Binding FirstName}" />
     <Span Text="{Binding LastName}"/>
   </FormattedString>
</Label.FormattedText>