I don't believe the type of splitter you are asking for exists.
USB-C Headphones contain an integrated DAC, which means the audio is sent to the headphones from your computer/phone over the USB protocol. You may already know that USB cannot be "split" like a headphone jack can, but you can use a USB hub to allow more than one USB device to be plugged into a single USB port.
You should be able to get both headphones to appear as separate sound devices on your computer if they are both plugged into a hub or separate USB ports. From there it will take some additional software to play audio to both devices at the same time.
One more caveat is that your computer is going to need a USB-C port or a very particular USB-A (3.1) port in order to support hubs with USB-C ports themselves.