If you are writing Morning Pages, and then working later on a project, you are already writing more than one idea. While I have found Morning Pages plus a project to be a satisfying amount of writing, I have also occasionally worked on still more. For my purposes, I find that it is the type of writing, more than the amount, which dictates my day’s work. I write Morning Pages, I write three pages of nonfiction, and then, if I still have the itch to write, I turn to fiction. And so it happens that while I wrote The Prosperous Heart, I also wrote a play. I remember this period as being both happy and productive, but I also remember that I myself was impressed by the sheer amount of work I was producing.
The best way to tell how much writing is “too much” is to experiment. Morning Pages? Yes, fine. Nonfiction? Also fine. Fiction? Perhaps a bit of a strain. In order to accomplish all three, I need to double my Artist Dates. Otherwise, my writing will become strained as I overfish my inner reservoir of images. Writing on multiple projects is not impossible, but it does take special care. Without such care, our writing becomes “thin.” It is my experience that multiple writing projects can leave one overtaxed.
It is best to write at several different times of day; Morning Pages in the morning, one project in the early afternoon, then perhaps a nap followed by work in the late afternoon or evening on the “extra” project. We need to be careful not to be greedy; after all, with daily writing, pages mount up swiftly.