Inclusive or.
Given a yearly probability of assassination of 1.6% for both (based on the assumption that both are as likely as a typical sitting president to be assassinated even though at most one will actually be president at a given time), and based on actuarial data, I think the "baseline" probability, before taking into account specific facts about either Trump or Biden, is around 47%.
Recent presidents have had super good life expectancy; all presidents who have died since 2000 were at least 90; and Carter is now 100). Both Trump and Biden are relatively rich, and don't (as far as I know) have serious acute health issues. I'm inclined to think that the above estimate is too high and it should be more like 30%, but I feel comfortable betting on <50%.