I don't know if I have the "right answer," but based on long, painful personal experience, I say: no. I do not believe it is possible for a person to be on the autism spectrum, and also have NPD.
My father is a full-blown NPD. I watched his descent into ever-more vicious madness over a period of 20 years, before I cut off all contact about 10 years ago. A book by Sam Vaknin, who himself has NPD, describes exactly how the NPD mind works. My father could have written it. It is striking how little individuality people with personality disorders have. As if they are all reading off the same script.
My BF, a guy I have known for 18 years, is on the spectrum. So was my grandfather. They are both good men - misunderstood men - who I love very much. To me, the contrast with NPD could not be more stark. There are moments when a statement or a behavior or a decision appears narcissistic, but any similarities are purely superficial. The thought processes behind them are completely different, and the long-term behavioral patterns do not match at all.
The reason I do not think Aspies can truly have NPD is because the neurological components necessary for it are missing. NPD requires full executive functioning, astute mind-reading capabilities, and highly developed emotional, social, and verbal skills. All of these are essential for manipulating and exploiting others to obtain narcissistic supply, i.e., emotional, psychological, or material gratification. That is their only concern, every minute of every day, which is spent plotting and scheming and avoiding any responsibilities or constraints. It warps their minds and makes them sadistic and diabolically evil, as well as boring and predictable (once you know that's what you're dealing with). Above all, NPDs absolutely require and depend on the presence of other people. They behave like locusts: they use people up, alienate or discard them, then go find another one.
With such different neurological architecture, it is hard for me to imagine how this could take root or develop in an autistic mind. (Thank goodness!)