You should ask both the dentist and orthodontist about recommendation letters first. Some dentists are not willing to write letters for students, so it's better that you check with them. To submit two recommendation letters, you should check with the schools that you want to apply. For ADEA AADSAS application, at most 4 individual letters can be submitted. However, most schools require letters from 3 professors, which means that you can only submit one letter from either the dentist or the orthodontist through AADSAS.
If you would like to submit both, some schools allow additional recommendation letters to be submitted directly to them. You can submit both letters by sending one of them directly to schools. But it's better to check with schools. They may have different requirements.