There is nothing in the rules strictly preventing it. So it isn't impossible. However if one has it out, and the other casts the spell, a different spirit would logically have to appear. Because find familiar says one shows up:
You gain the service of a familiar, a spirit that takes an animal form you choose: bat, cat, crab, frog (toad), hawk, lizard, octopus, owl, poisonous snake, fish (quipper), rat, raven, sea horse, spider, or weasel. Appearing in an unoccupied space within range, the familiar has the statistics of the chosen form, though it is a celestial, fey, or fiend (your choice) instead of a beast.
It also says that it stays where you tell it to go until it is unsummoned or put in its pocket dimension:
As an action, you can temporarily dismiss your familiar. It disappears into a pocket dimension where it awaits your summons. Alternatively, you can dismiss it forever. As an action while it is temporarily dismissed, you can cause it to reappear in any unoccupied space within 30 feet of you.
Thus, if one is summoned and obeys the first caster's commands, staying where the first one leaves it, the same familiar can't also be summoned by the second caster simultaneously. For NPCs, this shouldn't be a problem because they are both under your control. PCs shouldn't hope to try to share a familiar.
Also, note that NPCs also have access to the Familiar Variants that PC don't have access to. Though those don't use the find familiar spell.