Cooperative

In unpatched JJ2, Cooperative could only be played in local splitscreen, not online. JJ2+ fixes that, even allowing you to select difficulty (synced across all players) right from the main menu, and makes some balancing changes too.

"Single Player" and "Cooperative" are actually treated as separate gamemodes inside JJ2, and JJ2+ expands that distinction: Cooperative gives you polished gameplay that just works out of the box; Single Player gives you something closer to the unpatched JJ2 experience, but with fewer bugs. In particular, in Single Player (even in online servers) you can use cheat codes and debug mode, and dying resets the level, but in Cooperative, everything is synced properly, and dying mostly only resets your position. It’s technically possible (from the command line) to play Single Player with multiple local players, or to play Cooperative with only one player.

Deaths

In local SP/Coop games with more than one player, any players who run out of lives will now be revived (with no points or ammo) at the start of the next level, instead of being stuck seeing GAME OVER forever. If all players run out of lives in the same level, the Continue? screen will appear, with each player drawn separately.

In SP/Coop servers, the lives system has been removed entirely. 1ups are replaced with carrots, and dying never gives you a game over.

The most recently activated checkpoint is used as the start position for players who die or join the server. After a level begins, checkpoints in SP/Coop games may only be activated by local players.

Enemies

All enemies and bosses are carefully recoded to sync their health and movement for all players in a Cooperative server. (Unless scripted to have alternate behaviors.) The level will end after a boss is defeated, as you would expect.

Two concessions are made in Cooperative servers to make syncing possible: Uterus spike balls can only be damaged by bullets and do not fall down, and Bee and Rapier enemies use simplified attack patterns. They behave as normal in local games.

Miscellaneous

Any player collecting a coin causes every player to collect that coin, even spectators or players who haven't joined yet. This means everyone can beat levels that include mandatory coin warps. Coins do not reset when players die.

When servers cycle to secret levels like Gargoyle’s Lair, the next level setting will be changed to the level that normally follows the level containing the secret level's entrance. For example, a server would host first Medieval Kineval, then Gargoyle’s Lair, then Hare Scare.

Unlike all other pushable objects online, Big Box and Big Rock objects are solid in SP/Coop servers, because some levels make you stand on them to reach high areas. (Their positions are not synced between clients, though.)

It was technically possible to play online Cooperative in unpatched JJ2 if you manipulated the game’s memory, but it was very buggy. Some issues fixed from that version, which most players wouldn't have experienced:

• Clients now lose hearts when injured by spikes/enemies/etc.
• Levels actually end when clients reach End of Level events or defeat bosses. Clients actually agree with the server about whether the level is over or not.
• Levels with character-specific start positions (e.g. Knight Cap has different starts for Jazz and Spaz) now work properly.
• Score particles do not appear for points gained by non-local players.
• Beating levels from the original campaign now cycles to the proper next level. (There used to be some exceptions.) The final levels of fanmade episodes will generally cycle to Dungeon Dilemma (or Rabbit in Training on Easy difficulty).