Watchdogs 2 brings a new element into the watch dogs games by allowing you to team up with friends in order to complete co-op missions for extra rewards. It is possible to do these missions solo, but it will be much easier to do them with a friend with some coordination. The missions are randomly generated, so there's no one guide that will help you get through every one. But here's a list of the different types of missions you can get when you decide to go with a co-op mission.

Steal a vehicle from a restricted area.
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You want to steal a specific vehicle and make a clean get away so that thing is it mint condition when you get it to where it needs to be. Assigning one person to distraction duties while the other takes care of the car is a decent strategy.

Download some data from a restricted area.
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You'll need to stick fairly close to whatever it is you're downloading from while the data is transferred, but having both players in proximity to it will increase the download speed, so stay together to get this done as quickly as possible and get out again.

Steal one or more items from a restricted area.
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The items you'll need to steal will be scattered around the restricted area here, so split up and hunt them down to minimize the amount of time you are going to be spending in this restricted area.

Destroy multiple items in a restricted area.
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Unless you can get lucky and goad an enemy into throwing a handy grenade at least one of you will need need shock or explosive charges for these. Plan accordingly if you plan on picking up one of these missions.

Release a prisoner from a restricted area.
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The prisoner is always behind a door locked by a panel that requires physical access and is usually located some distance from the door, so a practical strategy is for one player to make their way to the prisoner whilst the other deals with the locks. Thankfully the prisoners don't need escorting after they're released, so as soon as they're untied you can beat a retreat.

Knowing the different missions should help you to figure out what it is you'll need to do in them to succeed and coordinate with your friend.