We have an Exchange Server cluster that we want to break into two standalone servers. What's the best way to do this?

Your best bet is to move the cluster's mailboxes to a separate server, then rebuild the cluster as two separate nodes. You can accomplish this by setting up a temporary server to hold the mailboxes or by removing one of the cluster nodes from the cluster, then reinstalling Exchange on each node as a standalone server. Microsoft doesn't support any other method; several cluster-specific flags and settings exist in Active Directory (AD) and are updated only when you decommission the cluster as a whole.