FirstViewModel.cs

public class FirstViewModel 
  : MvxViewModel
{
    private string _firstName;
    public string FirstName
    {
        get { return _firstName; }
        set { _firstName = value; RaisePropertyChanged(() => FirstName); RaisePropertyChanged(() => FullName); }
    }
 
    private string _lastName;
    public string LastName
    {
        get { return _lastName; }
        set { _lastName = value; RaisePropertyChanged(() => LastName); RaisePropertyChanged(() => FullName); }
    }
 
    public string FullName
    {
        get { return string.Format("{0} {1}", _firstName, _lastName); }
    }
}