In Too Deep

Players may turn face-down creatures face up while a spell with split second is on the stack.

If the resolution of a triggered ability involves casting a spell, that spell can't be cast if a spell with split second is on the stack.

Split second doesn't allow players to cast the spell it's on at times when they otherwise wouldn't be able to cast it. An enchantment with split second may still be cast only during its controller's main phase when the stack is empty.