One of the questions I was asked at a recent school visit was Can you write two books at the same time?
Yes I can, but they have to be of different genres. For example I can work on a picture book and a junior novel, or a picture book and a YA novel but NEVER a junior novel and a YA novel at the one time.
This is because I inhabit the setting of my story, I walk around inside it, familiarising myself with its smells and noises and peculiar atmosphere. Then I get inside the heads of all my characters, especially my protagonist. It would be totally confusing therefore to have two main characters in two different settings inhabiting my mind at the same time telling me what to do. Not to mention trying to follow the plotline of each novel.
How is it with other writers? Are you able to work on two books at once?