If a person committed an apostatizing saying or action and repented from it, then remembered another apostatizing action for which he did not recall repenting, does he have to remember both actions together and repent from them at once, or is his repentance for the first action valid, and his repentance for the second action separate?

This question stems from misgivings (waswasah), and you must dismiss these misgivings, strive against them, and pay them no mind. What benefits a person in repenting from sins, both what he knows and what he does not know, and what he remembers and what he does not remember, is general repentance, by intending to repent from all sins. A person does not exit Islam merely by misgivings or doubt about an act that would necessitate apostasy. Whoever's Islam has been established with certainty does not exit it except with certainty. You must rid yourself of these misgivings.