Split/Second

For this achievement you need to have filled your Power Bar up to Level 2 (Red marker) and use it to change the route of the track Twice (Red hexagon with arrow on it which appears at certain places on tracks) in one race.

This is fairly easy, i got this on my first race in Season mode, start off by drafting your opponents until you fill your Power Bar all the way and save it until you see the Route Change marker on screen, Press to use it. Now drift, draft, jump and avoid power plays until you earn your second Level 2 Power Play and wait for your moment to use again.

Note: Not all tracks have Two Route Changer locations in them, here are the ones i can see that have two (If i have missed any please leave a comment)
Airport Terminal (Two at the start/finish line and one just before the run way)
Ferry Wharf (One after the place with the hanging gas canisters and one just before the big red bridge)
Dry Docks (Two after the long straight [The Big Boat] will take two laps to activate the two and one at the big container ship at the start/finish) Easiest

Will unlock upon activation of the second route change.

Here's a video on Airport Terminal