According to this URL:
http://www.cisco.com/en/US/tech/tk583/tk372/technologies_white_paper09186a008018983e.shtml#dualhubs
Your issue may be using mGRE instead of p-pGRE tunnel interface types when both tunnel interfaces use the same source address. If you want to use mGRE tunnel intf types, use seperate source addresses.
If the same tunnel source is used for mGRE tunnel interfaces:
"This address is matched against the tunnel source ... address, but since both tunnels have the same tunnel source ... address, the first mGRE tunnel interface is always matched. This means that incoming multicast data packets may be associated with the wrong mGRE interface, breaking any dynamic routing protocol."
That may be the source of your issue.
With regards to OSPF: Is there a seperate backend connection between HubPrimary and HubBackup? That is, is one a DR and the other the backup DR for the ospf network?
The same doc suggests that using eigrp (or rip) may be easier than using ospf when running over an NBMA network. You may want to consider using EIGRP, instead of OSPF. I refer to the part of the URL that descibes what you have: Dual Hub and Dual DMVPN.
Let me know if this is of any help.
