Since no one has answered yet, I'll give you the practical answer.
You'll have issues with IPSec and static routing. "DMVPN" itself probably wouldn't have an issue, but it would depend on IPSec and routing to work.
It is easier, by far, to put in a second router. And when you factor in your time to try to make it work (and it may not work), the second router is less expensive.
Rob