One way maybe to setup the two ISP’s serviing these sites as BGP peers. I am not sure if or how this would work for sites in two different locations. I however do it with my two ISP’s coming into my single location were I have a Ethenet Fiber circuit from one ISP in one region and a T1 and ISP in another region. Now granted that traffic comes and goes over both connections, however a majority of the traffic comes in over one connection because routing favors it. But if the favored connection goes down then all traffic comes in over the other.
Another way to accomplish this that you might check into is with Global Load Balancing. There are load balancers to accomplish this. I use a load balancer (Coyote Equalizer) in my shop to load balance web, ftp and terminal services to multiple servers. Coyote has software (Envoy) that would Globably Load Balancer between two different Equalizers that are geographicaly disparate. The Equalizers can also be configured for duel redunancy for automatic failover as I do in my shop. I have two, primary and secondary, primary fails the secondary takes over rather quickly.
Hope this helps