Quite possibly being XO is an administrative job that may be held by various people aboard a Starfleet ship, and only select First Officers opt to take this as their "second proficiency". Spock instead chooses to be Chief Science Officer on the side, and Kirk's XO may be some Lieutenant or Ensign serving under Ben Finney...
Sorry, that's the most convoluted thing I've read in a long time, and completely at odds with everything we've seen both in Star Trek and throughout its various inspirations.
I suppose "Number One" is yet some third person.
And someone needs to mention (so I will) that Spock is not the first XO we've seen to have another job. Pike's XO manned the helm.