Item Duplication Glitch (Local Co-op Only)
Fair warning - unlike Time Traveling, which can be used for more harmless measures, this glitch will allow you to gain an extreme amount of money at an accelerated rate - which can render most progression on your island irrelevant, and may not go over well with your friends - please use this glitch at your own risk!
When playing Animal Crossing with a second player, using the Call Island Resident feature, there is currently a way to duplicate many kinds of items, and quickly sell an entire inventory's worth of valuable loot as fast as you can create more.
First, summon the second player using Call Resident, and then place a high-selling item on a small table or stool - this can range from small valuable items to rare bugs and fish, or even the Royal Crown sold at random times from the Able Sisters - which can sell for a huge amount of Bells!
Once the item is placed on the stool, have the second player (the one who cannot access their inventory) hold down A to grab the table and then spin the item. When this happens, the first player should grab the item with Y right as it begins to rotate.
If done correctly, the first player will pocket the item in their inventory, while the second player continues to spin a copy of the item on the table. If synced up, you can keep picking up duplicates until your inventory is full!
Source: https://gameplay.tips/guides/7166-animal-crossing-new-horizons.html