We helped a client of ours setup something akin to this, to work around their (essentially unfixable) Internet situation. We rolled our solution using OpenBSD/

We helped a client of ours setup something akin to this, to work around their (essentially unfixable) Internet situation. We rolled our solution using OpenBSD/pf/carp/pfsync due to client expertise and comfort with BSD. It works well enough that outages on their lines have shown up only as nagios alerts, not actual outages, even through many years of service.