Connect your second controller first and make sure it's linked to a different profile - this is crucial for everything to work properly. When you're ready press pause during any mission or free roam then look for the handshake symbol in the menu which takes you to cooperative options, then just select offline play and have your player 2 hit the confirm button. The timing here matters since you'll need to coordinate the confirmation between both controllers.

Watch for materials you collect during gameplay since they automatically sync between both players locally which is really nice for resource management. You've got flexibility in how you tackle objectives - either stick together for coordinated attacks or split up to handle different missions simultaneously, but keep in mind only player 1's campaign progress gets saved. Something interesting about the mission structure, you can position yourselves at different strongholds to divide and conquer more efficiently, especially during larger battles.

Just note there's no cross-region compatibility between different versions if you're thinking about online play instead - asia, north america, and europe servers are all separated. The split screen implementation works smoothly though once you've got everything set up properly, just make sure both profiles are ready before starting.