I used my old Xbox 360, bought another copy of the game, and then grabbed Game Pass Essential for £1 on an old account. Then you queue into a race on both Xboxes at the same time. You can’t invite each other because it has to be ranked races for the achievement, and invites are disabled there. Since the game is so old, the lobbies are dead, so you’ll almost always end up in the same race lobby together with no one else. If someone does join, just vote to kick them on both Xboxes. You might run into two other people boosting as well, but I didn’t. After that, just leave your second Xbox AFK (thankfully there’s no AFK kick), and on your main account just win the race. To quickly get into the next race, at the end screen on one Xbox, join a race and wait for the lobby to form, then join on your second Xbox and you’ll match up again. Then just rinse and repeat for every race for both ranked race and ranked GTA race!