how to implement double authentication

I am creating a small web app that needs two layers of "authentication".
The first layer is a secret code that must be entered to gain any sort of access to the site (no username is required). The second is once a user has supplied the secret code, they need to create an account.
On repeat visits users will enter the secret code and then login with their previously created accounts.
Any ideas how to do this while using the ASP .NET 2.0 Membership/Authentication framework?
Will it let a user authenticate twice like that?

Thanks
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