“I‘ve read about a split snake that lets you record and mix a live show at the same time. I was going to make my own by putting 16 XLR ‘Y’ adapters on my snake, but a guy told me that wouldn’t work. What’s up with this?”
Sometimes what you do not know won’t hurt you and sometimes it will. The words wouldn’t or doesn’t work imply an absolute state, which is not accurate here. It actually will work…sort of…depending upon your definition of “work.” Mic signals are very low level, and in many ways not as robust as line level signals. Microphones aren’t designed to be able to drive multiple inputs of varying impedances. Much like a guitar or bass the signal from a microphone can be changed by driving it into different loads. In fact, several high-end microphone preamps make use of this by employing a variable input impedance to allow the user to get different sounds from one mic. Further, depending upon the design of the mixers or preamps in question there is some chance that they will react with each other. Turn up the gain on mixer A and the signal appearing at mixer B suddenly loses all its high frequency content. In practice this is very unlikely with modern designs, but still illustrates the point.
Then there is the issue of grounding. When you ground the inputs of two mixers together like this you are most certainly creating a ground loop with the potential to cause problems. On the other hand, leaving a mic level signal input to a mixer without a ground is a recipe for other types of problems. It’s a bit of a Catch-22, but it can work. The best way to begin is with some experimentation using the gear in question. Buy or build your ‘Y’ adapter, hook up the two mixers, and start plugging in different mics. Compare the sound in each mixer against the sound when the mic is only connected to either of them at a time. If you get hum, try disconnecting the shield wire (pin 1 of the XLR) going into one of the mixers. You may well be able to get suitable performance. Modern equipment is often much more forgiving in these types of situations than the equipment people were using back when all of these conventions and rules came into being.
All that said, the ‘real’ or right way to split microphone signals is with transformers. Transformers provide isolation that will keep the components from reacting with each other in unpredictable ways. Of course, if you are concerned about quality you can’t use just any transformer. You really want to use good ones. These come at a price, but it does make a difference. Many users buy, build, or have a splitter snake custom built expressly for this purpose. The transformers are housed in the main snake box and there are two or more fans, or connectors for long snakes right there.