in router c you will have two seperate tunnels.
traffic will be routed based on your destination.
the tunnel will not come up until traffic is initiated from either end.
I do not know what type of routing scheme you are using so it is hard to say the type of routing protocol to recommend.
You could do this with default routes and weight the backup route if the primary ever went down, then all traffic would prefer the second link.
This is very basic but I think would do what you want.