I don’t know how many adults have the dual diagnosis of ADHD and autism, but it is becoming more and more common among kids, including my own son. When the new DSM-V was published a few years ago, it opened the door to having both diagnoses, which all prior versions prohibited. I suspect few adult have the dual diagnosis, just due to that.
This article is written about children, but the insights apply to adults too:
ADDitude Community Moderator, Author & Mentor on Parenting ADHD, Mom to teen w/ ADHD, LDs, and autism