Verified answer

You need one account 100% available to play online. Go to the Multiplayer -> Player Match Or combat training. Before doing anything else after you've done that, turn on the other controller for split screen, it will ask who ever is using the second controller to SIGN IN. Sign in AS A GUEST, NOT AS ANY OTHER PSN/GAMERTAG, Just as guest. It will show the second player's name as the first one, but with a (1) at the end.

From there, you can play Online, combat training or anything else. It works the same with zombies.