You present a hard edge case that isn't particularly well met by the current
infrastructure and it can't necessarily be done well or even done at all.
(Sean's tricky chocolate-consulting hack excluded). Probably the best
thing that be currently supported is getting two diverse connections to
a single provider that can globally aggregate your network. The connections
should go to different POPs and should follow seperate physical paths.
This should provide you the desired reliability and load sharing.
Thank you, thank you, thank you. This is exactly the kind of pragmatic
advice I was looking for. Saying "don't do it" or "multihoming costs" is
not helpful -- this sort of answer is.