Sharing Authentication between ASP.NET sites

I have two ASP.NET sites (they can not run in the same process) and I need to share authentication between them. If a user is in site A already authenticated and then goes to site B, I need to have a way to share this information with site B so the user is not asked to authenticate again. The same is true both ways. How do you share this information?

How can I limit the serving of a javascript file to only authenticated users?

I have a WebAPI 2 / AngularJS SPA application that uses Identity 2 for authentication. Locally my code stores a token for authentication. I would like to implement functionality that allows my application to request additional javascript for authenticated users after my initial index.html page has been downloaded.