The USB-C "splitter" to which you refer can't achieve what what you're attempting to do...
Each of your wired USB-C earbuds incorporates a DAC (Digital-to-Analogue Converter) that converts a digital audio stream to an audio signal that drives the speakers within the earbuds. In simple terms, your iPad can direct its digital audio stream to one USB audio device - not two.
If instead you use standard analogue earbuds that feature a 3.5mm plug - and a single USB-C DAC that has two analogue 3.5mm audio jacks - you can achieve your aim. As an alternative, if you use Apple AirPods, your iPad can wirelessly share audio to two sets of AirPods.
Share audio with AirPods and Beats headphones from iPhone or iPad - Apple Support