Two controllers required for thisFrom the main menu enter Multiplayer and select Splitscreen, the game will ask you to press
on your second controller.
Select which event type and make the event short.
I suggest a race off so you can unlock one of the following easily:
The Hard Charger achievement in DiRT Showdown worth 19 pointsUse a full tank of boost in one go without crashing in a Race-Off event Select the Miami Track and set it to 1 Lap in the Track Selection, once the event is complete the achievement will pop at the loading screen.
For me both players had to finish the race but people have commented saying that retiring counts.
Here's a video