Hello Tayeb,
are you suing two DMVPN cloulds = two mGRE tunnels on each spoke routers or both hubs are in a single cloud (single mGRE tunnel on spokes).
if the tunnels are two tuning is easy and requires just to play with ip ospf cost so that some spokes point to Hub1.
Otherwise you need to tweak OSPF over the single GRE tunnel:
you need to move to a different OSPF network type:
point-to-multipoint non-broadcast
this peculiar OSPF network type allows to use the command
neigh x.x.x.x cost w
in this way you can specify different costs for different neighbors and this should allow you to implement some form of load-balancing
But this is something that you should test in lab, there can be some issues to be solved so I recommend lab testing of proposed solution.
Hope to help
Giuseppe