One pattern often seen in the family with an NPD parent with more than one child is very different treatment of the children - one child is typically chosen as the "Golden Child" who can do no wrong, whom the narcissist grooms, bribes, indulges relentlessly to shape the child in the parent's own 'golden image'. The other child or children are treated very very differently - the Scapegoat has a terrible time in these families, growing up (though they usually emerge into adulthood as the ones who can see what happened, the Golden Children generally go on just the same as the older narcissist/s in the family, to recreate the scenario and damage a lot of lives along the way).
So, Dianthus, your question is can a person be both autistic and a narcissist? Given the intense and obsessive shaping that NPDs do on a Golden Child, (often the first born, not always though), the answer is theoretically yes; however in real life, narcissists don't want children whom others might view as flawed in any way - the world outside the family has to be shown a show of a perfect child to feed the parent's need for narcissistic supply. The parent uses the child's "perfection" as a self-reflection, much more than just the run of the mill parent likes to take pride in their children's achievements. Possibly a very HFA child chosen and cast and moulded in the classic Golden Child role could be both autistic and narcissistic.
But NPDs are actors, dissemblers, liars, relentlessly calculating and performing, creating and acting out a false persona to the world.. not many people on the spectrum can do that for any length of time.
So to your question, personally I think it is possible, yes, but so rare as to be unrepresentative. That's my answer, anyway.