Who should be the main doctor, primary or oncologist? Aunt is passing on tx. Should doctors be communicating with each other about care?

Choosing not to have active treatment for cancer is a reasonable choice, but it is not 'doing nothing.' Depending on her cancer and stage of disease, her PCP may be able to manager her care for a time. See if there is a palliative care team at the same hospital where the oncologist and PCP are affiliated. They are the experts on comfort care. Referral to hospice can come from PCP.
Also, if you are not already her health care decision maker (health care proxy, Power of Attorney for health care, etc) please get this in place soon. Your aunt stays in charge until an MD involved in her care says that she cannot make her own decisions. Usually elders ask their POA to be part of medical appointments, to better understand what the situation is and what your aunt wants.

With straight Medicare I don't think you need a referral but with Medicare Advantages and some secondary insurances, they may require a referral. Without it, they may not pay. So its usually seeing your PCP first and he refers you to a specialists depending on what tests reveal. Some MAs are HMOs and they are notorious for referrals. So make sure if you need a referral by calling your insurance provider.

In my experience with my dads wife once she had the cancer diagnosis her oncologist basically became her cancer primary so coordinated everything and all the cancer related doctors, tests, surgeon, radiation, rehab… Her primary continued to coordinate everything else and received notes from the oncologist regularly. They also communicated when they needed to, for instance when an issue crossed both domain’s but she tended to go to her primary for things first because she had a longer relationship with him and was more comfortable, he would just send her to the oncologist when that was warranted.

With a DX of cancer, I'd see the oncologist for that and anything arising from it (i.e., metastasis), but you can take your mother to her primary or specialists for other conditions.

This is obviously my personal opinion; we went to specialists and found it much more expeditious and time saving than going through PCPs, some of whom eventually suggest specialists b/c the issues were beyond their scope of knowledge and practice.

With cancer of any kind, you don't need to waste time with someone who can't address that and the issues arising from it.   And not to challenge JoAnn's advice, but my sister never had to go through PCPs to get to an oncologist, but that was likely b/c her medical insurance didn't require it. 

And for you and the frustration, anxiety and concern that arise, try to locate a local Gilda's Club to meet and interact with other cancer patients as well as their families and caregivers.

You go to the PCP first. Before a oncologist gets involved the PCP will give a referral to a gastroenterologist to do a colonoscopy to determine how far the cancer has spread. If your Aunt has already seen a gastroenterologist, then she needs referral from him to see an oncologist. If she refuses to see the oncologist, ask her if she is ready for hospice. I am not beyond putting a little scare in these stubborn ones. I would tell her if she plans on doing nothing, then maybe she should go into a NH. Because its not fair to expect your mother to care for someone who is too stubborn to do anything about something that is curable if caught early.

Usually, there is no consulting between a PCP and Specialists. Letters are written telling the PCP what the Specialist is doing concerning the patients care. Once a patient is in a Specialist hands, decisions are made between the patient and the Specialist. The patient can always consult with the PCP or get a second opinion.