Did some more controlled testing and find flurry and accuracy tributes both work. Glad they're worthwhile.Thanks for the reminder Brohg. CoF3 (Benefit Flurry III) increases chance by 6% which parses true enough. Flurry rates improve by stacking both double and triple attack chances with it. I measured up to a 48% increase in flurry rates stacking FoC3 + Stoic Wolf Familiar (6% doubles) + Pureforge Rnk III (50% triples). The rate of a 4th swing seems to be about twice that of a 5th swing, measured with a 1h weapon to avoid Knight's Advantage extra 2H swings.
The wolf familiar can be used again since the Item Effect AA changes that blocked it based on HP regen. It's higher chance to double attack then ranger Predator (4%) so it can grant 2% more doubles over that buff (because highest one counts). I don't know of any other group buff that improves this chance. I used the wolf for testing. There are other more powerful familiars to use normally like Emperor Ganak's.
It's not about stacking. I think it's not functional because all players have been 100% double attacking already for the past decade+.