Again, to be all the way clear: You seriously can't play "A Way Out" without two humans controlling each of the two player characters.
The game can be played cooperatively over the internet, or on a single game console/PC. Bonus: You only need one copy to play the game, even over the internet on two separate game consoles!
If you're playing locally, like I did, you'll see a screen divided in two: On the left is Vincent, on the right is Leo. I played as Vincent, my buddy as Leo.
Sometimes they're in the same area, doing different things or working together, and sometimes they're in completely different places — no matter what, each character can directly impact the other. Early on in the game, this plays out in obvious ways: Vincent helps Leo climb up to a ledge, for instance. As the game goes on, though, the concept of two different screens and two different characters becomes something far more interesting.
Notably, the controls are never too complex — most of the game's actions come with on-screen prompts, which should help make it a bit more accessible to sporadic game players.