Second Guess can’t be cast unless the spell it targets is the second spell cast this turn.

It doesn’t matter if the first spell cast that turn has resolved yet or not. It also doesn’t matter how many spells have been cast after the second one, as long as the second spell cast is still on the stack.

Most spells and abilities that copy a spell create that copy on the stack. These copies are not cast and won’t count toward the number of spells cast that turn. However, a few (like Isochron Scepter’s ability) instruct you to copy a spell and “cast the copy.” These spells are cast and will count toward the number of spells cast that turn.