Will the achievement unlock for:
- Both of us?
- One of us?
- Neither of us?
I can confirm (as others have stated) that the answer is "neither of you." I played the entire campaign on the Xbox 360 with a partner, including the Calibration Course. We did not get the achievement during this playthrough. It requires that one player be finished with co-op and the other have never played it.
If you're on the Xbox 360, you can "cheat" the system. If you and your friend both finish the co-op campaign, you can each create a new profile on the console, and then each of you can use your new profile to play with your partner's old profile. Note that I tried this in split screen mode on the Xbox 360 and it did not work - I had to do it via Live with another Xbox 360.
Since this requires XBL Gold on the Xbox, if you only have one Gold-enabled account on your Xbox, you can use a 24-hour code (that comes with many online-enabled games, I have a stack of 4 or 5 personally) to get your second account online. Also, if you create a new Xbox Live Silver account on your new profile, you'll be offered a 1-month Gold trial. You can use this trial to create your "dummy" account, but note that this trial feature is limited to 3 trials total per console.
On the PS3, this might be easier, as there's no additional cost for playing online. This won't work on the PC, where the game is attached to your Steam account.