You need two Nintendo DSes, two copies of the game since you will be farming both files. A cooperative buddy will also be helpful. The process goes as follows:

Player one needs to go through their game and note which days Tom Nook buys turnips (or some other commodity) at a high price. When you locate a day that you will turn a good profit from selling turnips, have player two with the second DS go to a Sunday morning at 6 AM. (The second player may need to change the system settings to do this).

Player two needs to find Joan and then buy as many turnips as can be afforded. Connect the DSes and have Player two sell the turnips to Nook's store in Player one's town. Repeat this farming technique as often as needed.