Have totally crumbled on the Nano front. I wasn't liking my characters or their story (although I think there might be something there), and then I received a 'get out of jail free' email from the editor at HMB with her comments on my chapter and a request for an update on progress. So I reckon that's a legitimate reason to put the Nano novel aside and go back to the previous story.
On the other hand, an artist friend of mine sometimes has four paintings on the go simultaneously. Is it possible to work on two novels at the same time? Do you, can you, do this?