How do I get double cash mode?

It is a special game mode called Bananza only available on Steam or mobile, not the online flash (which is the one that you might be playing). The mobile version has special Club Arenas where special rules may apply depending on the game mode. Bananza's rules are: start with 500 eco instead of the regular 250, sending bloons give twice as much eco, and Banana Farms, Cobras, Monkey Aces, Monkey Engineers, and Heli Pilots give twice as much money (for Banana Farms: $120 instead of $60, Cobra: $180 from Wired Funds instead of $80, Monkey Aces: twice as much money from supply drops, Engineers: twice as much money from bloontraps, Heli Pilots). In order to play bananza, you'll have to download the app or play it on Steam.