This achievement requires you to ride on a mountain together with a friend. At any point in the game press
and go to 'MULTIPLAYER'. Click 'LIVE RIDE' and invite a friend.
Unfortunately not many people own this game on TA, so finding a friend with this game can be quite difficult. During my playtime (up to the completion) i offered to help TA-members to get this achievement. This worked great for llRuGGeDKeViNll, JJBDude48 and BigD The Chief, however, as i deleted this game all i can say now is good luck! ;)