Feel free to add coop support to NSMB yourself
I am pretty sure the work behind this project is way more than you'd expect
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Youtube - GitHub - NSMB Central

Sorry for the bump but I figured this might be helpful for shadey or TheGameratorT when/if they come back or if the source code is released and someone is ambitious.

Firstly thank you so much, I've got so much enjoyment from this with my sibling over the past three years who is now 9 and we have got every Star Coin and Secret Exit possible in this hack. Feel free to ignore this post if you do not care because I don't want to sound ungrateful. I never thought something like this would be possible and I'd love to understand the wizardry behind this.

But now to the point, I would just list the bugs that we found along the way. I've tried to make it very clear and easy to read. Note when issues were encountered, I tried DSONE EOS, Wood, YSMenu and nds-bootstrap/B4DS to fix them but generally made no difference.

W2-4 Secret Exit
* Problem - flag does not spawn and exit cannot be completed.

All cannons
* Problem - freeze when a player enters.
* Fix - changing to DSi Mode if playing in TWiLightMenu++.

W7 boss
* Problem - freeze with horizontal blue stripes scanning across screen or a red screen crash in TWiLightMenu++.
* Fix - changing to DSi Mode with TWiLightMenu++ fixed the issue.

W4-5 final star coin
* Problem - Need to break bricks with bob-ombs.
* Normal behaviour - A pipe spawning bob-ombs normally hangs over this site, but here, no bob-ombs come out.
* Steps taken to try address issue - we tried many times to carry a bob-omb from elsewhere but it would seem like we would always barely miss.
* Fix - maybe place a bob-omb closer to where the pipe leading to the third star coin is.

W4-6 final star coin
* Problem - Need to hit a red switch above the raft.
* Normal behaviour - In single player, can normally stand on Dorrie's head but since a raft is used, it cannot be reached.
* Steps taken to try address issue - we tried using a mini mushroom and wall jumping, but again would barely miss.
* Fix - maybe another platform or bringing Dorrie back.

W8-8 second/third star coin
* Problem - blocks do not break with the fruit bomb things and rocks which break blocks only fall in the first quarter of the level.
* Steps taken to try address issue - dying at the checkpoint did not help.
* Fix - change brick blocks to empty brown ones?

Final level
* Problem - second switch that turns the level upside down doesn't do anything. Therefore cannot go through upside down door and cannot finish the level.

Occasional crashes and desyncs
In some levels, death meant no return. eg. W8-8, w7-5
W6-6 has less time for some reason.

There are some other issues, but these are the major ones which impede progress.

Just kind of curious if there's any way to bring the other player back once they have no lives