Operation: Tango

If it's your first time playing, you won't get access to the world map until after finishing the first mission. But if one of the players has already progressed past that point, the world map will open straight away to choose a mission.

Both players must move their cursors to Montreal, Canada on the map and press A. If you haven't done the achievement before there will be nothing there, but you'll know if you've hit it when your player icon appears. Once both players have hit it, the achievement will unlock.

Both players have to click on Montreal on the Mission Select screen. I'll upload a screenshot when I get a chance.