[GRRM is asked about the pros and cons of writing a parallel book.]
I don't necessarily recommend the whole "parallel books" thing. I've done it before, with Wild Cards volumes 6 and 7, and to a lesser extent volumes 4 and 5, and there's some cool things you can do... but all in all, I would have preferred to do it all in one book. There were special circumstances behind the split of DANCE from FEAST, as previously related.