DPM Team blogged in the beginning of the month about secondary DPM protection in a Exchange 2010 DAG.
First, there are a few basic ground rules when talking about DPM protection for Exchange DAGs:
1. Only one node can be protected by an DPM server. For example, in a three node DAG, if DPM1 installs an agent on Node1, no other DPM server can install an agent on Node 1. However; any other DPM server can install agents on Node2 or Node3. In a second example; If DPM1 has agents on Node1 and Node2, then Node3 will be the only node that another DPM server can install an agent on.
2. DPM does not care if the mailbox database is active or passive.