This story in the Times mentions some high flying debut novelists whose second attempts have failed to live up to their first: Charles Frazier and Zadie Smith (who more than made up for it with the superb On Beauty in my opinion) for example. Included in the list is Donna Tartt's The Little Friend, which I would strongly argue does not deserve to be on a list of failures. Sure, she had the syndrome bad probably, which is why it took her so long to write (ten years between outings) but I thought it was very good book - a very different book, but a good one nonetheless.
Of course all of this has come to my attention because I am working on my own second novel after a reasonably well received first novel. I am constantly being asked by people if I feel under pressure to live up to my first. My answer is that of course I do, but it's not other people's expectations that I feel acutely, but my own. I just want to write a better book. I am writing a better book (she says, hoping that statement won't come back to bite her on the behind!). It's been 7 years since I started The Sound of Butterflies - I should be a better writer by now. So, yes intense pressure, thanks very much, but I don't think it is to do with Second Novel Syndrome. I hope I never rest on my laurels and feel released from that kind of pressure, because that's when I have stopped trying to improve.
All a writer can do when faced with something like SNS is to ignore it and sit down and write the best book he or she can write. And that goes for third, fourth and fifth novels.