Python 2 and 3 are in most practical senses two very closely related, but different, languages. So your scenario is not very different from the two groups of developers suggesting Python and Ruby.
By the way, assuming either language satisfies the external requirements for the project, any half-way competent boss would say "Figure it out. Bring me your decision by the end of the day(/week/whatever). KTHXBYE."
By the way, assuming either language satisfies the external requirements for the project, any half-way competent boss would say "Figure it out. Bring me your decision by the end of the day(/week/whatever). KTHXBYE."