How often could a person have two cancers concurrently?

Fairly often: This is common in autopsy practice. Often one is unknown, or the presence of two masses has caused puzzlement. I recall one case frm my early years with primary tupes in four different organs. I've formed the impression that this happens a bit more than would be expected by random chance or common exposures (i.e., a heavy smoker), but i can't prove it. I hope this helps.