My experience playing with splitting a signal between a couple of 200 LED strips was that there was enough level drop or interference introduced by a Y-cable splitter to cause intermittent weird behavior. (We’re pretty much at radio frequencies – does the Y cause reflections or standing waves that occasionally add up to data errors? If it’s not plain voltage drop, would a little filtering on each leg of the Y help?)
Anyway, that project is returned to the queue for the moment, but I was thinking about trying something like this SPI Amplifier and splitter next time instead of the Y cable (unless anybody knows of something cheaper/better!)