Conditional on one site or the other going down, the second instance adds little buffer.
An ufai would simply focus its efforts on pieces of code likely to be common between the two sites, ensuring that it can take both down at the same cost. This also assumes that developing such an attack is costly, which it may not be: I would expect a sensory modality for code to reduce our commonly made coding blunders to the level of “my coffee cup is leaking because there’s a second hole at the bottom”.
Conditional on one site or the other going down, the second instance adds little buffer.
An ufai would simply focus its efforts on pieces of code likely to be common between the two sites, ensuring that it can take both down at the same cost. This also assumes that developing such an attack is costly, which it may not be: I would expect a sensory modality for code to reduce our commonly made coding blunders to the level of “my coffee cup is leaking because there’s a second hole at the bottom”.