Enigma Drake's power is equal to the number of instant and sorcery cards in your graveyard.
If one has Spring//Mind (or some other Aftermath card) in their graveyard, does that grant 1 or 2 power? Does it matter if both halves have the same or different types, e.g. two Instants vs an Instant + a Sorcery)?
It becomes a 1/4.
Spring // Mind is both a sorcery card and an instant card, but it's still only one card, and Enigma Drake counts cards.
You're actually asking about split cards —the fact that they also have Aftermath is irrelevant— and the following ruling is found attached to Enigma Drake:
A split card only counts once for Enigma Drake’s ability, even if it’s both an instant and a sorcery.
