Depending on what you mean by clustering, I've posted at least once or twice about my quick-and-dirty way of allowing two DA servers to manage the same domains by using different DNS settings in the different DA systems, and by using each DA machine to be one of the two DNS servers for that domain.
You can do rsync based replication for your web content, host e-mail on one of the systems, and perhaps do a backup MX record, and have a single server acting as a MySQL slave, or do MySQL replication between the two nodes. Not that DA necessarily supports that out of the box, but its more than doable.