[√] Duskmantle Seer reveal a split card

708.2. In every zone except the stack, split cards have two sets of characteristics and two converted mana costs. As long as a split card is a spell on the stack, only the characteristics of the half being cast exist. The other half’s characteristics are treated as though they didn’t exist.

708.5. An effect that asks for a particular characteristic of a split card while it’s in a zone other than the stack gets two answers (one for each of the split card’s two halves).