I have been working as the main chairside assistant for about two years now.
It's important to understand that when you work together with another assistant the dynamics are totally different.
Usually, Doctors have two assistants. One is expected to take on more responsibilities, in that case, a RDA. The other has the duty of preparation of work station, instruments, and materials. Those are not followed at all times though, specially when plans can change quite often at a dental office. Both assistants duties end up being important the same.
Let's say the procedure was a common composite filling and it ended up turning into a BIG cavity with a pulp exposed! Instruments and materials will probably now be exchange. Will the RDA be waiting on the DA to set-up again? No. Both will communicate and "fix" the situation together. The important rule to remember is: WORK AS A TEAM. Don't have the mindset of "this is not my job". A good dental office team is the one with a mindset of "these are all our jobs". If one employee is free, he/she should ask what do to help.
Also, remember dentists retain the authority to determine which new duties their staff members can perform and on which patients they are performed. In the case of a DA, the required supervision follows the statute, but for a RDA, general or direct supervision are determined by the supervisor (doctor or a Hygienist).