Bosses - All bosses now function correctly with 2 players. Defeat them together!

World Map - Play every single level on every single World with a friend!

Respawning - Players can now respawn if the other player is still alive. Simply press A while dead to join back in! (Disabled during boss fights).

Shared Coins - Players can collect 100 coins to both earn a 1-up.

Separate Lives - Mario & Luigi now have separate lives.

Updated Collisions - Players can walk through each other.

2 Player Support - All mechanics that grant players rewards such as Red Coins and Toad Houses now grant both players rewards.

Many other tweaks & fixes!

How to play:

1). Download the patch here (v1.01).
2). Apply the patch to a clean US rom of New Super Mario Bros .DS.
3). Launch the game on 2 DS's and both select the same save file.
4). Connect with the other player.
5). Play!


Potential Issues:

Desyncs - Occasionally, the game can desync. Although the game has been tested and every level has been played multiple times, it is possible not all desyncs have been found. If you experience a desync, please post where it occurred so it can be fixed for future updates.
Reloading to the last save will resolve the desync in most cases. If saving occurred during the desync, the save file may become unplayable.

Minor Issues:

Slowdowns/Lag - The game can slowdown at certain levels. Levels were not edited when possible to ensure the game is as close to the original Single Player mode as possible. Staying close to the other player can reduce slowdowns.

No Mario VS Luigi mode - This mode is unavailable in this hack.

Missing Sound Effects - Some minor sound effects are missing occasionally.

Missing Sprites - A handful of sprites are missing due to incompatibility issues with Multiplayer.

FAQ:

Does this work on Flashcart/Android/TWiLight/MelonDS etc.? - Any software/program that can load a .nds file is capable of loading the game but the system/emulator needs to be able to connect to another player.
This means the game can only be played on a real DS (through a Flashcart, TWiLightMenu etc.) or on an emulator that is capable of emulating wireless (MelonDS is the only DS emulator capable of this).

Does this work on a US copy of NSMBDS? - This only works on a US copy.

How do I get to World 4/7? - Beat the boss on World 2/5 while both players are mini.

Special Thanks:

 TheGameratorT - Helping me debug and figure out a ton of the multiplayer functionality, particularly with the World Map. This hack would not have been possible without his help so make sure to check out his work!

Ed_IT - Answering general hacking questions & providing updated hacking tools.

 KingYoshi +  Ndymario - Beta testing early versions of the hack.

 Arisotura - MelonDS made testing and debugging so much easier. Amazing emulator.

Before this misleads anyone, this is just a heavy enough modification of the Mario vs Luigi mode rather than an actual co-op hack.

Multiplayer hack of New Super Mario Bros.
Also, excuse the very unoriginal name, it's only a placeholder.

Correct me if I'm wrong but there are no co-op hacks on this board so this hack may be a nice change for those looking for something different. This hack changes the versus mode into a cooperative mode. The objective is to reach the pipe at the end of each level. There are no world maps, saving or any of the single player features, it is simply a hack to play a few levels together. The levels aren't exactly complex but I do plan on updating the hack over time with some more features/improvements. Although rare, the games can de-sync but most of the time it works fine.

Screenshots:




Features:

- 3D Goombas.
- 4 new multiplayer levels.
- No battle stars.
- Some new tilesets.
- A new rocky/grassy background.
- Bowser Jr boss battle.
- Coins have no function.
- Changed music (still limited due to the Mario vs Luigi mode only loading 2 songs).
- Only "Co-op" mode can be selected at the menu. (Thanks to  skawo)


How to Play:

- Download the following xdelta patch here and apply it to a CLEAN US ROM.
- Connect both DS's to each other (DLP will not work).
- Select the following options (Grassland stage NEEDS to be selected or the game will not function properly).





Known Issues:

- Sometimes at the start of the Bowser Jr fight, Mario can become stuck and unable to move. I don't know what exactly causes this but if Mario moves before Luigi does, this bug is usually avoided.
- If one player is entering a pipe while another player loses their last life, the game may hang.
- Spikeballs can occasionally cause a small amount of desyncing (Needs to be tested further).
- Some missing sound effects.
- Possibly other issues.


Special Thanks:

-  Dirbaio (Use of his no sprite limitation ASM hack, which can be found here).
-  skawo (Answering many of my questions & some ASM hacking).

Mario now has HP!
Could be handy if anybody wants to make a hack with a similar HP system to SM64.

Video
https://www.youtube.com/watch?v=FeT47bRFq3g

Screenshots


Notes:
- The player cannot become Small Mario.
- The player can collect coins to restore HP (Default: 15 coins = +1HP).
- HP has a cap (Default: 5HP).
- Taking damage while Big Mario will result in losing some HP (Default: -1HP).
- Taking damage while in Fire & Shell will result in losing the power up, HP remains unchanged.
- Taking damage while Mini will still result in Mario dying, regardless of HP. Because of this, the player may now press Select to return to Big Mario.

Download: HPMode.cpp

Only "Mario vs Luigi" is selectable in the main menu.

All in Overlay 9. Credit to  skawo

The hack is now fully released, please see the first post for more details!

I'm not having any luck with this, just a white screen when I load the ROM. Maybe it's my flashcard or maybe I did something wrong but I don't think I did. Have u got this to work?

Might be a dumb question but how did you go about finding those values  skawo?
For example, how did you find that time was "5C2042E2002081E5"?
Was it through the debugged No$gba?

Could you explain how you go about doing it? I've looked for a few tutorials on here and on google but no luck.

Not that I can remember.

Bowser Jr does seem to attack Luigi too. The sprites in this hack seem to function normally. If you want me to test any sprites, feel free to PM me.

Thanks, it's building fine now.
For anyone having trouble, just follow this little tutorial. It helped me out.
http://melonds.kuribo64.net/board/thread.php?pid=73#73

I was doing some debugging on Mario vs Luigi and stumbled across this.
If you force the time value to 0, the game will actually end and this will appear. The values do not display the wins of each player, they just seem to be set to 0.
I don't remember seeing this text anywhere in the game either.

That works, but it seems to kill all other music and the majority of other sound effects.

Eventually.

It's quite messy and unfinished at the moment but once I clean it up, I'll release it.



True, but then those tiles can get in the player's way.

I'll look into that. It's pretty much impossible to get a mega mushroom outside of the first level anyways.

Thank you for this find! Turns out, the patch released was faulty. It was excluding vital code that excludes some sprites that can cause a desync. I have updated the patch to fix this issue.

Anyone using the previous version (v1.00) should download the new patch now! (v1.01)