You don' t need a secondary tunnel from each of the remote locations. If they are configured to use the head office 300C as their default gateway then once you have a second link on the 300C the remote locations will be good to go.
It' s easy to add a backup WAN link - just plug it into WAN2, configure a second default route under Router->Static Route, and give it a different priority than the main link (click the advanced button when adding the route).
You can also use Policy routes to force specific traffic out specific interfaces if you like.