I'll lay it out for you brotha.

21 STP during WS with a 504 delay GA like Perdu Voulge (13.7 base TP) from Brutal, Rajas, and /SAM gives:
16.5 TP from the main swing
17.7 TP if you land one extra hit
18.9 TP if you land two extra hits
20.1 TP if you land three extra hits
21.3 TP if you land four extra hits (2 DAs)

I did a lot of Raging Rushes in Abyssea with 34% Double Attack and 0% Triple Attack during Visions of Abyssea and never saw a 3-DA return (should have been a 3% chance with capped Acc).

Myth busted.