BloonyFox made this hack, in Spanish (v5.0) NesDraug translated it to English and released a 20 lives addendum.
Some say Bloony's patch changes too much stuff from the game. If you want a more vanilla SMW with co-op there's an unrelated by rhelmot.
Version 0.0.1 https://github.com/rhelmot/smw-coop
Keep in mind though, rhelmot stopped releasing updates after the very first release and there's things that may not work so good, see co-op.txt
Below the changes made by Bloony's hack:
Important Changes
The map and level colors have been reverted to the game's original style.
The difficulty curve has been improved to make the game more enjoyable.
Mario and Luigi can now perform a Spin Jump while carrying objects.
Mario can slide down slopes while carrying objects, just like Luigi.
Luigi jumps higher than Mario to add gameplay variety.
Levels have been extended to enhance the gaming experience.
Semi-solid floors have been darkened to make them more noticeable.
The title screen now displays the completion percentage instead of the number of exits.
Pressing START on the map will bring up an option to save the game.
When performing a Spin Jump on an enemy, you bounce like in Mario Maker.
In dark levels, the black outline of sprites has been changed to gray to distinguish them from the background.
The box where an item is stored has finally been fixed with a patch.
Overworld improved.
Texts have been better written.
Changes
Credits updated with a better design.
Major changes in Bowser's Castle.
Shading has been added to some textures.
The texture of the Hopping Fire has been changed to a different style.
Invisible blocks now show a slight shine to indicate their location.
The graphic for the cape suit has been changed because it previously looked like the bee mushroom.
Mario now displays raccoon ears when wearing the suit.
The forest ground has been changed to its prototype version to add more color to the environment.
Title screen improvements.
Issues in "Yoshi's Island 3" have been fixed.
P-Switch graphic updated to match Super Mario Maker.
The [?] blocks now work correctly in haunted houses.
The pipe in "Donut Secret 1" has been fixed.
Muncher graphic updated to match Super Mario Maker.
Backgrounds changed due to bugs... Sorry :'v
The transition between maps is now slightly different.
Koopa masks have been changed to Bomberman faces.
After completing the special level, Bullet Bills are now skulls instead of birds.
Goomba has been replaced with the prototype Goomba graphic.
Some Super Mario Maker blocks have been added.
Mountain environment added to the game.
The overworld can no longer have glitched colors like in creepypastas.
Some colors have been improved.
The crushers in the castles now have better graphics.
New explosion effect.
Shells now have an animation when waking up.
The list has been improved to make it shorter and less boring.
The checkpoint and castle ladder animations have been fixed.
I spend my days following the ROM hacking scene, searching for cool hacks and translations of old games that were left in Japan. All translation and hacking credits go to their respective authors. Neither I nor anyone at CDR translates games.