This is a world championship season with another player over xbox live, you can either be in the same team as each other and atempt "There's No "I" in Team" together or race for different teams and battle it out. You must win the drivers championship though. If you need a partner someone can be found in the trading thread here.
This can be done in one season using the method from last year which is this:
Both players should trade wins back and forth to keep the points very close to each other. Now, on the last race, have the host quit out, and the other player will finish the race to get the achievement. Once this is done, invite the other player back in, and let the host finish the race in first, making sure to win by enough points to beat the other human player.