You need an original ROM file, and this here, according to the article.
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How would they even retrofit that…? Maybe it could do splitscreen through something like Steam Play Together, at least.
Nintendo 64 emulation with netplay has existed for 20 years.
20 earth years?!? :O
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These modifications are possible because the game has been decompiled back to modifiable source code. At this point they can add anything to the game with enough dedication.