The SEAT drivers took the top four positions in both races as Tarquini led Rickard Rydell, Jordi Gene and race one winner Yvan Muller over the line in the second race.
The race started behind the safety car on a track flooded by a violent thunderstorm with Felix Porteiro on pole.
In the early stages the BMW contingent of Porteiro, Andy Priaulx, Sergio Hernandez and Augusto Farfus set the pace but soon the SEAT tide became unstoppable.
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On the straight that leads to turn four on the fourth lap, Italian Tarquini made his move, squeezing between Priaulx and Hernandez, and outbraking them amid the water puddles.
"I had the advantage to be fastest out of the corner, but it was a bit risky," he admitted but the risk paid off and became the springboard to his victory.
Joerg Mueller and Augusto Farfus were the best of the BMW men in fifth and sixth.
Dropped by BMW and demoted to Independent, Porteiro took his own revenge by finishing seventh ahead of Tom Coronel plus Priaulx and Hernandez who completed the top ten.
Tarquini and Muller head the standings with 15 points apiece with team-mates Rydell and Gene hot on their heels with 14 with Mexico the next stop on the calendar in a fortnight.
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