ASP.NET Core Pages Handler

You can separate the login and register forms into two different partial views. This way, each form will have its own isolated scope and validation context.

Here's my working demo based on this PageModel:

public class RegisterOrLoginModel : PageModel
{
    public class RegisterModel
    {
        [Required]
        public string UserName { get; set; }
        [Required]
        public string Password { get; set; }
    }
    public class LoginModel
    {
        [Required]
        public string UserName { get; set; }
        [Required]
        public string Password { get; set; }
    }
}

Here's are the partials showing how to pass the relevant class to its @model directive:

_RegisterPartial.cshtml

@model RegisterOrLoginModel.RegisterModel
<h4>Register</h4>
<form method="post" asp-page-handler="Register">
    <input asp-for="UserName" /><br />
    <input asp-for="Password" /><br />
    <button>Submit</button>
</form>

_LoginPartial.cshtml

@model RegisterOrLoginModel.LoginModel
<h4>Log In</h4>
<form method="post" asp-page-handler="Login">
    <input asp-for="UserName" /><br />
    <input asp-for="Password" /><br />
    <button>Submit</button>
</form>

Include both partial in the Razor page like this:

<partial name="_LoginPartial" model="new RegisterOrLoginModel.LoginModel()" />
<partial name="_RegisterPartial" model="new RegisterOrLoginModel.RegisterModel()" />

Then, instead of having two bound properties in your PageModel, update your handlers to take the respective model as a parameter. Then ModelState will only work with the parameter and will not be affected by other stuff:

public void OnPostRegister(RegisterModel model)
{
    if(ModelState.IsValid)
    {
        // ...
    }
}
public void OnPostLogin(LoginModel model)
{
    if (ModelState.IsValid)
    {
        // ...
    }
}

Alternatively, create separate pages for each form.