One way to do this is for two ISPs to band together in order that each ISP can sell half of a joint multihoming service. Each ISP would set aside a subset of their IP address space to be used by many such multihomed customers. Each ISP would announce the subset from their neighbor's space which means that there would be two new DFZ prefixes to cover many multihomed customers.

Each multihomed customer would run BGP using a private AS number selected from a joint numbering plan. This facilitates failover if one circuit goes down but doesn't consume unneccesary public resources per customer. [...]

I've heard of this from others as well. It seems to be technically feasible, but I am curious about the social aspect: would ISPs actually do this? Would customer's find it acceptable? (given it still locks them to an ISP, now to two of them.) In fact, this is technically feasible right now with IPv4. Does anyone know of a pair of ISPs doing this? John