Why is the split termination method required for the CAN bus?
It helps with the real-world. 

A perfect differential pair, could be terminated perfectly by a single resistor.

The inevitable imperfections mean that there is benefit to making each line appear to be "terminated individually".

The illustrious Dr Johnson explains it better than I:

https://www.sigcon.com/Pubs/edn/DifferentialTermination.htm