Book in a Week. This sounds good for those of us who can't stick with writing a novel for a month. (By way of Becky Levine)
Focus On One Writing Project Until You're Done (Procrastinating Writers). I actually do do this when I'm working on a big project, particularly if I've moved well along with it. I find that if I'm deeply involved with something, I'll continue to "work" on it even when I'm not working. By which I mean, if I've been working for weeks on something, at the end of the day while I'm doing something else, ideas will continue to come to me. I'll get ideas for the project while driving in the car. So, yes, I find that focusing on one writing project can be very helpful. More break-out experiences. More flow states.
On the other hand, if you spend two years on one project and then have trouble selling it, you may be sorry.