This is easiest to do in Horde and can be done on Wave 1. With a co-op partner, start a game of Horde. Have your partner kill one or two enemies to accumulate a small amount of gold then get downed. In the meantime, kill enemies yourself to collect more than double the gold your partner collected (communicate with them to make sure) then get downed yourself. When the timer runs out for both players, the game will end and, provided you collected at least double the gold, the achievement will unlock. Repeat this for the other person(s) in your group.