I can confirm this is really doable with one person who has two xboxes (long story) and two copies of the game. Choose a strong team like Mclaren or Red Bull, and keep the other two teams at bay race by race. Since your "boosting partner" is an idle team mate, it'll be disqualified before the end of the first lap which means you'll only score 25 points for the win.
The key to getting all three co-op achievements in one season is making sure you block your closest rivals in the standings at the start of the race so they lose positions, but just enough that you get a warning and don't get disqualified yourself. Also make sure the AI is set to the the easiest difficulty when you start the season, that way you'll make up the positions again without too much trouble and take the win. You need to try make sure that your main rival never scores more than 25 with both cars. Most importantly, don't crash into anyone, you'll often end up 2nd best AND have a 10 sec penalty for every incident... Rather use the blocking tactic, but at a slow speed and not too long as not to be disqualified.
Hope that helps any peeps like me who couldn't be bothered with the admin of a live co-op partner.