Kyle Gann wrote on his blog Post Classic about his Summer Projects, three new pieces. It seems he's finished with two and nearly with the third. The amazing thing to him is he was writing all three at the same time. You Go Gann!!!
Mozart wrote his final three symphonies while working on an opera. We don't know why he wrote these symphonies, but we do know he finished all of them within approximately a month of initially putting pen to paper. Bach wrote so much music it is impossible to image he wasn't writing multiple pieces pretty much constantly. So, writing multiple pieces of music simultaneously is not new. What is new -for Gann and composers like me - is the ability to think clearly about multiple pieces and keeping them straight.
I have successfully composed multiple pieces at the same time in the past, but not with any regularity or consistent success. It is something I continually strive to do, work on consciously, because I feel the process itself does something to the brain --I think differently about music when multiple pieces are swimming (successfully) in my head. And all the pieces end up as better pieces.
What happens when it works is a fermenting of ideas from one to another, without confusing issues or themes. When it isn't successful, good ideas tend to migrate to one composition while the other(s) suffer. It is rather like absorbing good ideas from studying Mahler, Mendelssohn and Machaut at the same time (again, not an easy task, nor one I recommend for those wishing to remain sane). The end result is a collection of richer ideas (richer compositions).
I'm attempting to do the same thing as Gann this Summer. I've completed a 10 minute orchestral work Exchanging Glances, an Oboe Concerto, a Piano Sonata, a string quartet and an opera (well, I'm re-working the opera so perhaps it doesn't really fit in the "new composition category"). The ideas have remained separate - thanks to a composition book that lives with me. When new thoughts come to me (whether in my dreams, on the road or when I'm working on something else), I jot them down into the book. If something rings vaguely familiar the other compositions are also in the book for easy reference. Sometimes these means new ideas can be scratched out when duplicated, or morphed into better ideas when met with previous concepts from other compositional ideas.
It isn't a perfect system. If it was I'd be successful at continually writing multiple pieces at one time. It is helping...
So, here's Gann. Congrats on your success. May it continue to bear fruit (and I can't wait to hear the results)! And to the rest of you composing music - keep the ideas flowing. The world needs more music!