Never

Each split card has two names. If an effect instructs you to choose a card name, you may choose one, but not both.

If another effect allows you to cast a split card with aftermath from any zone other than a graveyard, you can't cast the half with aftermath.

While not on the stack, the characteristics of a split card are the combination of its two halves. For example, Destined // Lead is a green and black card, it is both an instant card and a sorcery card, and its mana value is 6. This means that if an effect allows you to cast a card with mana value 2 from your hand, you can't cast Destined. This is a change from the previous rules for split cards.