Well, what you would actually want to do is have server 2 be a domain controller as well. By adding it to the same domain and promoting it to a domain controller (and also as a global catalog server) - you can essentially have users authenticate locally. Because they are in the same domain and are both domain controllers, they will be synchronizing all of the active directory database information automatically for you. So you would then be able to manage all users and their access from your one location.
I would definitely recommend a VPN for this.
You can also take advantage of Microsoft's Distributed File System to make each server a backup of the other's data files - and also set it up so that when a user logs into either location, their files are accessible to them locally from any desktop.
I would suggest you take a class in Administering Windows Server (insert version here) - these usually tell you how to configure things properly. Or find a book on such, there are many good productions out there. Microsoft Press is decent. And then you'll have a reference on how to do a lot of stuff. Others might be able to suggest other resources you can get for helping you manage and explain how all this works.