You don't need a second client license to run against more than one Windows 2008 server, but you do need another server license for the second server itself.
It sounds like you're simply wanting to set up what used to be referred to as a Backup Domain Controller (back in the Windows NT 4.0 days-- 1999 and before) and is now called installing an additional domain controller (since domain controllers are equal peers and there's no longer a concept of "primary" and "backup"). When you install Windows 2008 on the first computer, you'll want to create a new Active Directory Domain if you don't currently have one, and set it as a Domain Controller. When you install the second server, you'll want to set it to be a DC as well. They will replicate user and computer accounts, Group Policies, and other things in the AD store between them. If you want redundancy such that either can "run" the domain, you'll want DNS servers on both of them, as well. Make the second DC uses the first one for its primary DNS server in the network settings, and have the first DC set to its own IP to avoid ending up with 2 separate (and potentially conflicting) sets of DNS entries, and have both DNS servers recursively query an outside DNS server, such as Google, OpenDNS, or the servers assigned to you by your ISP, if they don't find the address on your network. Be sure to set these 2 as the DNS Servers for your clients in their network properties as well.
Lastly, with regard to file shares on Windows, real-time duplication is more expensive and complicated, but possible with things like clustering. However, if real-time isn't critical, there are many ways to copy data between hosts -- robocopy is a good and efficent program for doing this.