MVC Nested View Model with Validation

I'm trying to put login and register form into same view. I did everything suggested in other questions but my problem still not fixed.

Here is my parent view authentication.cshtml:

@model Eriene.Mvc.Models.AccountVM
    <div class="row">
        <div class="col-md-6">
            @Html.Partial("_Login", Model.Login ?? new Eriene.Mvc.Models.LoginVM())
        </div>
        <div class="col-md-6">
            @Html.Partial("_Register", Model.Register  ?? new Eriene.Mvc.Models.RegisterVM())
        </div>
    </div>

In my partials I use forms like this:

@using (Html.BeginForm("Register", "Account", FormMethod.Post, new { @id = "login-form", @role = "form", @class = "login-form cf-style-1" }))

One of the actions is like this:

[HttpPost]
[AllowAnonymous]
public ActionResult Register(RegisterVM registerVM)
{
    if (ModelState.IsValid)
    {
        User user = new Data.User();
        user.Email = registerVM.Email;
        user.ActivationCode = Guid.NewGuid().ToString();
        user.FirstName = registerVM.FirstName;
        user.LastName = registerVM.LastName;
        user.Password = PasswordHelper.CreateHash(registerVM.Password);
        return RedirectToAction("Index", "Home");
    }

    return View("Authentication", new AccountVM() { Register = registerVM });
}

And here are the models I'm using:

public class AccountVM
{
    public LoginVM Login { get; set; }
    public RegisterVM Register { get; set; }
}

public class RegisterVM
{
    [Required]
    public string Email { get; set; }

    [Required]
    public string FirstName { get; internal set; }

    [Required]
    public string LastName { get; internal set; }

    [Required]
    public string Password { get; internal set; }

    [Compare]
    public string PasswordRetype { get; internal set; }
}

public class LoginVM
{
    [Required]
    public string Email { get; set; }

    [Required]
    public string Password { get; set; }

    public bool RememberMe { get; set; }
}

In the action registerVM's Email property has value, but others are null. ModelState.IsValid is false. What am I doing wrong?