If the analog sticks are not responding correctly, Hori recommends the following solution in the manual for the Split Pad Pro Attachment Set:
[quote|format=featured]Disconnect and connect the controller from the Nintendo Switch or Nintendo Switch Dock and turn ON the Nintendo Switch. Once Nintendo Switch is ON, rotate the analog stick 2~3 times. The process above will only work when this product is attached to the Nintendo Switch™ console and the power supply is on.[/quote]
The Split Pad Pro Attachment Set is not compatible with the Nintendo Switch calibration menu.[br]
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In the case that the accessory's dedicated recalibration option does not work, it's possible that you have control stick drift. The solution in that case is the cleaning of the potentiometers or a replacement of the control stick module. A reference for disassembly is provided [guide|132760|here|new_window=true].
I hope that your problem is solved in a quick and simple manner.