if you decide to boost the online achievements
It is impossible to boost without "sacrifying" the form level of a chosen player.
If you boost with 2 players, at each race, the winner gets +1, the loser gets -1.
So alternating winning will not work, and leave you at same level without progression.
One of you will have to finish at top level, the other one at the bottom.
Run 99 races with always the same winner,
(he will get Start, Good Form, Out of Reach, World Beater)
then run 99 races with the other player winning all,
(he will get Start, Giant Killer, Good Form, Out of Reach, World Beater)
the run a race with the first player as a winner again.
(he will get Giant Killer)
At the end player 1 will be of form 98, and player 2 of form 2.
For each half of the session,
only the player that needs to win requires to be at his controller.
The other one can go away from his xbox/tv.
The player to win starts a private elimination match with no AI, any track.
(elimination will only last ~1 minute)
He invites the other player. The other player accepts the invite.
From there for all the levelling of player 1, no interaction of player 2 is necessary.
Once player 1 at the top, swap and repeat.
Do not forget to swap again for one race at the end, for the last cheevo.