To obtain this achievement you need to give a co-op partner $500,000 in one transfer. The game allows you to transfer 20% of your total money at one time, so you will need to amass $2,500,000 in your own bank first. Then you click on the icon on the right side and under the coin tab you’ll see your co-op partners. In the box under their name you can see the amount you will be sending when you donate dollars. This amount should read >$500k. Send it to your partner and the achievement is yours.
To make it easier you can have your coop partners send you money and you can send it back to them. Set up a lot of banana farms and upgrade them. It will still take you a lot of rounds to make that much money.