Can you cast a spell with split second after another spell with split second is already on the stack?

Split second (CR 702.61) states that while a spell with split second is on the stack, players cannot cast other spells or activate abilities that are not mana abilities. This restriction applies to actions taken while that split second spell is waiting to resolve — it does not retroactively prevent spells already on the stack, nor does it block a second split second spell from being cast on top of it.

Here is the key point: split second only locks the stack after the spell with split second is placed on the stack. If a second split second spell is cast on top of the first before the first resolves — which is impossible in practice because once the first is on the stack you cannot cast more spells — both cannot legally coexist via normal casting. However, if by some effect a spell is put onto the stack without being cast (e.g., via a copy effect or a spell that puts another spell onto the stack), split second does not interfere with that either, per CR 702.61b.

In practical terms: once a split second spell is on the stack, you cannot cast a second split second spell on top of it, because casting requires you to cast a spell, which is exactly what split second prohibits. The answer to the question as asked is therefore No — you cannot cast a new spell with split second (or any spell) after a split second spell is already on the stack and waiting to resolve.

Concrete example: Your opponent casts Sudden Shock (which has split second). You cannot respond by casting Krosan Grip (which also has split second), because split second on Sudden Shock prevents you from casting any spell in response. Krosan Grip simply cannot go on the stack on top of Sudden Shock.

Citations: CR 702.61a states "As long as this spell is on the stack, players can't cast other spells or activate abilities that aren't mana abilities." CR 702.61b clarifies that split second doesn't stop triggered abilities from triggering or state-based actions from being checked.