In order to obtain the elusive achievement you'll need to get the x2 power up back to back. Once obtained the achievement will take a few seconds to pop. The best settings will be: Easy difficulty(as it gives you more time to pick up the next power up), a 4player game in party mode, select Fruit Bomb(preferred) or Dodgeball.
In my experience on Fruit bomb just fly around and disregard the monkeys and go for the power ups. Odd enough it seems like all were x2's but who knows what you'll get. You may also do this on hard difficulty if you are going for gold medals.
In Dodgeball move your characters to the middle and just run around in circles or wait for the power ups to appear. I'll be honest, in here the x2's do not appear as often but it's an alternative if fruitbomb isn't working for you.