I looked at an article about extending the default asp.net mvc 5 Identity Model for register and login here
My question is, how can I merge the default login system with my own entity framework code first database with the ability to extend the login system too, should I use two different databases for login and other functions? It seems the whole thing about extending the asp.net mvc 5 login system with own database is very complicated even though most of the articles I see suggest otherwise...there is just not enough documentation and implementing a complete new login system is not a solution for me because there's security concerns
Have a look for examples of using ASP.NET Identity without using the Entity Framework for example: https://code.msdn.microsoft.com/Simple-Aspnet-Identiy-Core-7475a961
This will show you how to use the basic authentication, where you can plugin your own validation to your own database.