Hello Azm
First, you should ensure that your underlying DMVPN dual hub single cloud setup is correctly configured. In your scenario, you have redundant links at the spokes, and since we are using a single mGRE tunnel, you must create a single tunnel interface on each spoke that uses a loopback interface as the source tunnel.
Once that is done, in order to ensure the proper redundancy, you will have to ensure that the loopack address is reachable via the routing protocol you use. From there on, you simply have to employ a routing protocol that will correctly route traffic as you desire. OSPF may not be the best choice because of the fact that you want to prefer the MPLS networks over the Internet. As demonstrated in the lesson itself, it’s difficult to use OSPF to prefer one hub over another.
EIGRP may be a better option as it gives you more flexibility in preferring one hub or path over another. Ultimately, the best way f examining the behavior of such a topology is to test it out in a lab scenario. If you do so, keep us posted as to your progress…
I hope this has been helpful!
Laz