These twins are from the 7th league, "Double Damage," where you have to complete their demands, Forward Mine 5 Cars, Nitro-ram 5 cars, Get 10 Bonus Hits in Destruction, and Wreck One car and finish 1st in Event 6.
As you beat the twins, you unlock the mod Scatter Shot, which fires all three Bolts at once.
Ayumi and Harumi looks similar, same hair style, but different clothing. Ayumi has white clothes, and Harumi has dark clothes.
They both have the same car, the Nissan Skyline GT-R Nismo Z-Tune. Ayumi's is yellow and white, and Harumi's is black and pink.
When you beat them, you get the Double Trouble achievement.