No, there is nothing stopping state police from requesting ATSB to investigate, but there is nothing stopping me from requesting an election either, it's just that neither of us will get what we want. The Act states - (1) Subject to section 21, the ATSB is not subject to direction from anyone in relation to the performance of its functions or the exercise of its powers. (The prescribed exception to this comes in writing from the Minister).
Basically, yes, the ATSB could share information if they are also conducting an investigation, but the fact is, they won't.
The reason the ATSB will always say 'no' to such a request is that to participate in a police investigation would compromise the aim of the Act under which they operate, which stipulates that ATSB investigations are not undertaken to apportion blame or liability to any person. If they start peforming police investigations, they lose their image of impartiality and therefore hinder their investigative abilities. Witnesses, for example, will not provide information as freely when they are aware that their testimony may be used in criminal or civil proceedings. Oh, and even when providing evidence in coroner's court, ATSB evidence comes with conditions and protection. As per the information re Commonwealth juristiction, a federal entity such as ATSB are not answerable to a state authority such as the police.
I'll leave it at that - I've probably waffled enough considering the real issue here the tragic loss of two souls from our flying fraternity. May they RIP.