Getting Chests & Help
There are two ways to get multi-lock chests: hunting them in hidden areas or through combat. In regular areas, multi-lock chests spawn randomly at the same points that regular chests do. You'll need multiple players to open them (either 2 or 4 depending on the number of locks). To get chests from combat, you need to be fighting with two or more people. You will never encounter a chest from combat that you don't have enough people to open. And be careful - if someone exits the battle area, you may not be able to open a combat chest in time.
Everyone in the battle, regardless of whether or not they opened the chest, gets doubloons equal to the number of pirates in battle, so don't worry if you're fighting with three players and a two-lock chest appears.
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Different types of "chest routes" can have different respawn times, and this happens quite randomly. You may open chest A first, then B, and B will respawn first. You may open a group chest only to have a regular chest spawn immediately after. Consider switching realms and checking different locations to maximize your chest opening.