I'm listing mine in both sections. I have a similar situation, where I'm a volunteer in a free dental clinic, assisting dentists and dental students with procedures. So I'm entering the experience in both the "Extra-curricular/Volunteer/Community Service" section of the AADSAS as well as in the "Dentistry Experience" section.
I'm not splitting up the hours, since they both count as volunteer experience AND dentistry experience...so you should be allowed to put it in both sections. Under "Dentistry Experience", you have the option to choose "volunteer" from the drop-down box.
If they have any questions, they can easily ask about it during the interview. But I think it's accurate to list it in both sections without splitting up the hours, since I was BOTH a volunteer and getting shadowing/observation of dentists all at once.
EDIT: Just spoke to an admissions adviser at one of the dental schools I'm thinking about and she told me to DEFINITELY put it at least in the dentistry experience section, but she doesn't mind if I also include it in the volunteer/community service section. This is just one school, but I'm guessing others feel the same.