I’m going to use this dual aux-fan mod for the P1 Series. I will go with option 3, which is to connect the two fans together using a cable splitter, however I have not been able to find a JST MX 1.25mm 4-pin splitter yet. Are there any splitters out there?
If I understand you correctly, you want to perform a simple wire splitting? Is that correct?
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Splitting the fan cables using a simple wire splitter is an excellent methodology for transforming your printer into a $500 paperweight.
The tip off should be the 4 wires and this instructions if you read the fine print in the MakerWorld links. It’s buried so you would be forgiven if you missed it.
Since this is a PWM fan, the Pulse Width Modulation and Tach/Sense signals control the fan motor. They are digital signals not analog and cannot simply be split along the same wire as a power cable can. For that, you will need a multi-fan controller. As the documentation in the Makerworld link shows, the P1 Controller board already has a second fan controller and that is what is recommended.
However, if I were a tinkerer–which I am 
–and I were so motivated, I’d pull a page out of the multifan PC builder playbook. There are many multifan solutions out there, all you have to do is use Google. Having said that, almost all of them are for 5VDC or 12VDC fans.
Noctua is probably your best option considering that they are maker friendly and provide a more robust(safe) solution. They can also provide you with considerable tech support in this endeavor not to mention all the YouTube videos out there that will aid in how to implement a fan controller inside a PC, just apply that to the printer.
You will have to find a space–or maybe print an enclosure–to hide/mount the controller. Note: There are much cheaper solutions on Ali Express that are far less trustworthy and it is up to your electrical engineering expertise to puzzle-out how to use them, they rarely come with instructions or if they do, they are either in Chinese or broken-English.
Then all you need do is source a JST to Dupont 4pin cable to do the translation. Those are too easy to find on the web so I won’t waste time here. Personally, I prefer to make my own since I won’t pay the exorbitant prices charged for prewired but there is something to be said for buying something off the shelf.
Here’s a search on Amazon for pigtailed cables that will at least allow you to avoid having to crimp you’re own. You will have to solder the two ends together but if you’re already this deep into an electronics level mod, what’s a little soldering amongst friends. 
https://www.amazon.com/s?k=4pin+JST+to+4pin+dupont+cable
Good luck 
and let us know how you make out. 
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