This is an issue for pathologists if they are happening concurrently to name it. If one follows the other time-wise, which sometimes happens in tobacco users the are second primaries. They develop after getting through oral cancer, a second primary. Second primary may also refer to a concurrent cancer of a different origin / etiology. The metastasis of oral cancer to the cervical nodes, to the lungs to the brain, are all known sequential developments and have been seen happing for many decades, those are a spread ( metastasis of the original disease). It is ot a black and white terminology but therse are the various ways that people have described it to me in a clinical setting.

I tend to think that continuation and spread of the existing disease is a met, and anything new that develops is a second primary if the causative agent is different or the timing is not concurrent.