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”.
Within the confines of the story:
No star that has been visited by starline has ever been seen from another, which implies a vastly larger universe than can be seen from a given lightcone. Basically, granting the slightly cryptographic assumption that travel between stars is impossible.
The weapon is truly effective: works as advertised.
Any disagreement with that would have to say why “”“ ‘Assume there is no god, then...’ “But there is a god!” “”” fallacy doesn’t apply here.
The threat of a nova feels like a more interesting avenue than the mere detonation.