What are you backing up ? If it is a permanent connection between two sites you could do it as follows:-
Permanent connection as primary, VPN (I assume) across DSL as secondaries. You could either use static routes to provide the backup or run a dynamic routing protocol across the sites and prefer the primary connection while it is available. N.B. - You will have to encapsulate the routing protocol traffic into a GRE tunnel across the VPN.
Does that help?
