Scoring a million points on a song in co-op mode is insanely hard, you'll need to be extremely good at the game and you'll also need to know someone locally who is too.
It's only obtainable on 2 songs*, those songs being Jessica and Freebird, now it's up to you which song you choose to tackle this achievement on but before you do decide let me give you some information.
Jessica is a lot shorter than Freebird but it's margin for error is minimal, you'll really need to nail this song and you'll both need to be on expert.
Tto date the highest score achieved for it is 1.1 million.
Freebird, this song is incredibly long and it can be very boring to play over and over when trying to reach a million points, however, with the songs length comes a massive margin of error, the high score for this song to date is nearly 1.4 million with both players on expert, to make Freebird even more enticing, you can both play on hard and if you play well enough you'll still manage to score the required amount, if you don't have a friend local who is great but you are (or vice versa), one player can play on expert while the other can play medium, even with this set up you can still manage a million points.
Of course, if you're both great at the game and can handle it, play on expert to guarantee yourself the million points!
I've linked in a video of 2 guys playing on expert and hitting over a million points to give you an idea of when to use star power, try to follow this star power path and so long as you don't mess up loads you'll manage the achievement!*referenced from the http://scorehero.com high scores, GH2 co-op mode.*Credit to Kov and KoT for the video