Two completely independently developed C compilers, or independent implementations of Common Lisp are much more compatible with each other than are Python 2 and Python 3.
They could as well have different names (think Perl vs. Rakudo).
Racket is more compatible with Scheme than Python 3 is with Python 2, yet has a different name, for good reasons.
Both Python 2 and Python 3 could respectively use a simple versioning numbering scheme. Python 3 version 1, version 2, version 3, ...