You can use Distributed File System (DFS) to maintain an offline store of all your user data that will still be updated via replication when changes occur.
To do this, simply create two shares on separate servers (the names are not important).
Now, in Active Directory, create a DFS Root.
Then, create a DFS Link that points to one of the shares you have created. Add a replica to that DFS Link and point it to the other share that you made (use the automatic replication option).
Now, instead of mapping the users home directories (or shared directories) to the share itself, map it through the DFS Link.
To remove the worry of data integrity during replication (you will need to read up on why that is an issue), switch your secondary DFS Link Replica to the offline status. It will still receive all of the replication but users are unable to access it. So when your main server crashes you can switch the secondary DFS Link Replica to online and users will be able to continue working with minimal interruption.