public class ExampleController : Sitecore.Mvc.Controllers.SitecoreController
{
    public override ActionResult Index()
    {
        return View("Index");
    }

    [HttpPost]
    public ActionResult Index(string formPostData)
    {
        ViewBag.SaveForLater = formPostData;
        return Index();
    }
}

@using Sitecore.Mvc
@using (Html.BeginRouteForm(Sitecore.Mvc.Configuration.MvcSettings.SitecoreRouteName, FormMethod.Post))
{
    @Html.AntiForgeryToken()
    var term = ViewBag.SaveForLater as string;
    if (!string.IsNullOrEmpty(term))
    {
        <p>Submitted: @term</p>
    }
    <p>
        @Html.Sitecore().FormHandler("Example", "Index")
        <input type="text" name="formPostData" placeholder="Enter something" />
        <input type="submit" name="submit" value="Search" />
    </p>
}

@model Website.Models.TestViewModel

@using (Html.BeginForm())
{
    @Html.LabelFor(model => model.Text)
    @Html.TextBoxFor(model => model.Text)

    <input type="submit" value="submit" />

    <input type="hidden" name="fhController" value="TestController" />
    <input type="hidden" name="fhAction" value="Index" />
}

namespace Website.Models
{
    public class TestViewModel
    {
        public string Text { get; set; }
    }
}

public class ValidateFormHandler : ActionMethodSelectorAttribute
{
    public override bool IsValidForRequest(ControllerContext controllerContext, MethodInfo methodInfo)
    {
        var controller = controllerContext.HttpContext.Request.Form["fhController"];
        var action = controllerContext.HttpContext.Request.Form["fhAction"];

        return !string.IsNullOrWhiteSpace(controller)
            && !string.IsNullOrWhiteSpace(action)
            && controller == controllerContext.Controller.GetType().Name
            && methodInfo.Name == action;
    }
}

namespace Website.Controllers
{
    public class TestController : Controller
    {
        public ActionResult Index()
        {
            return View();
        }

        [HttpPost]
        [ValidateFormHandler]
        public ActionResult Index(TestViewModel model)
        {
            return View(model);
        }
    }
}

public class MainViewModel
{
    public LoginViewModel LoginModel { get; set; }

    public RegisterViewModel RegisterModel { get; set; }
}

public class RegisterViewModel
{
 // Your model properties
   }

public class LoginViewModel
{
  // Your model properties
}

public ActionResult MainView()
    {

        MainViewModel model = new MainViewModel
        {
            LoginModel = new LoginViewModel(),
            RegisterModel = new RegisterViewModel()
        };

        return View(model);
    }

@Html.Partial("_Login", Model.LoginModel)
  @Html.Partial("_Register", Model.RegisterModel)