Opinions for my project

I want to make a project which I have divided into 2 parts.

First is ASP.net website and second part is Windows form application, both applications are connected to same database, user can go, and view data/information on website and some of the users who want just specific part of the whole application can use the Windows form application.

Users who use Windows form application can make offline changes which will be sync to online database later when connected to Internet and their data will be available on ASP.net website.

I want to make architecture in a way that I don't need to write separate code for both applications like if I am going to make login on both apps than the class which contain the method to validate user login can be used for both apps. How and what scenario should I follow for implementation?

And one more thing, Windows forms application may have multiple instances for single login (user).

Is there already an architecture design available that I can follow?