As you know, you can protect/backup a DPM server using a secondary DPM server. You can see the details on the following TechNet article:
With the recent release of DPM 2010, I have seen a lot questions about protecting DPM 2007 using a DPM 2010 server and vice versa. Anyway, the simple answer is both primary and secondary DPM severs should be running the same version.
•DPM 2007 Server cannot be protected by a secondary DPM 2010 server
•DPM 2010 Server cannot be protected by a secondary DPM 2007 server