So, theories and the like about this weapon come here.
My theory is that there was once a single sword - the primordial sword, ZERO Calibur. A powerful force, one that created the galaxy of Serenth (which has no name in-universe until after LS), split it into two swords when it deemed the sword too powerful. And so, Excalibur and Whycalibur came into being.
This is a copy-paste.
ZERO Calibur was themed around "0" (the number) rather than the letter "Z", unlike its two components.