R2 and Back Row Attack - Whistle?

[Antonio.King]

Since the second referee is responsible for identifying illegal attacks and illegal blocks just as much as the first referee, they should be prepared to whistle this fault if the first referee does not.

Key words, "if the first referee does not". If they have, or have begun to blow their whistle indicating this fault, then the second referee does not have to blow their whistle.

This boils down to a solid pre-match discussion. Often times, you'll hear the first referee ask for the chance to blow their whistle on the fault first, prior to the second referee's whistle. If the first referee does not acknowledge your discreet signal, as the second referee, blow your whistle.