Does a BNC splitter introduce defects into the signal?
While I don't expect any nonlinear effects that would change the (dominant) signal frequency, the reflections at each interface on your cable will introduce a set of delayed, attenuated copies to your signal.

Whether or not that is bad for your application can't be answered from the data you give.

As a general comment:


  I'm currently doing some contract work that includes finding and recommending a signal generator model for frequency measurement device verification.


Doesn't sound like you are the perfect person to do that. Maybe look for a sub-contractor with experience in handling of delicate measurement references. That's not something you want to botch yourself.