Firstly for this achievement you will have to unlock the "Duel" emote, which requires you to win 15 hands in Tournament Poker. Tournament Poker is a VIP only (Level 5 or above) tournament that is located in the centre of the upper level of the Casino.
To enter a Poker tournament, you can either buy into Level 1 for $25,000 or try your luck at winning the raffle by buying Cocktails (at the bar - Straight ahead of you upon entry to the Casino) and gaining entry into the related level of a Tournament.
Once you have won 15 hands in Tournament Poker, you will have unlocked the ability to use the "Duel" emote.
Now you can search for any type of 2-player table and use this emote at the same time as your opponent.
To find someone to help you get this achievement, please use the Achievement Trading Thread located here