How can the actions of an autistic / low IQ person be categorized into autistic behavior or IQ behavior?

That's a difficult question actually. The answer comes not from any specific set of criteria, but from clinicians with long experience in working with the mentally handicapped or autistic population. Like anything else in medicine, if you see enough of what "normal" is for a particular population, abnormal will stick out.

While it is true that many of the low IQ children will have behavior problems, it is often more attention-seeking behavior, rather than antisocial or attention-avoiding behavior. Autism tends to be more antisocial. Downs syndrome is a good example of a low IQ population that is generallly happy and cooperative. You can also have the opposite extreme of an autistic savant, whos is brilliant but unable to function normally in society. It's nice to have clear cut examples like this, but the reality is that many children with autism also have lower IQ's, so it is often difficult to point to a certain behavior and assign it to a specific condition.

Behavior is a complex interaction between genetics and environment, and I'm not sure your question will ever be answered in a black and white manner.