Hoping someone can answer this.

If I play a game with a friend in co-op and then play another game with another friend in co-op (2 friends! I know...) will I have seperate stuff in each, like upgraded guns and so-on, or is there a persistent character where it keeps track of everything?

Just curious.

I... have no clue.

I think there is only one save, so you started a game with your other friend I think you would lose the progress with the first friend.

About to find out in a few minutes when I get back from work, but a friend is saying that everytime you start a game you can choose a specific chapter and you'll keep all your weapon upgrades etc. Hopefully I can confirm this later for anyone that is still curious.

Edit: I can confirm that every time the game saves, you keep all your ammo, weapons, health sprays and upgrades, so effectively you can play with several different people and always keep your same loadout, meaning that you can take your fully upgraded weapons into a game with someone playing it for the first time. Awesome.