I have started writing two separate books simultaneously - a practice I tell my students to avoid vehemently. It is important to be able to focus on a single project rather than trying to juggle the cognition necessary to create multiple worlds of reality that have nothing to do with one another. I didn't start writing two simultaneously because I'm a hypocrite who doesn't listen to his own years and years (and years uncountable) of technical and creative training. I started it because I realized that my love of stories and the circumstances of my life are inextricably linked.
This shouldn't be a grand revelation. How many author's personal struggles have defined their bodies of work? Pretty much any one of them who is worth a damn.
I would not consider myself a multi-tasker. In fact, I am not convinced that anyone can truly multitask (feel free to prove me wrong). So, when I have 90 different things happening in my mind, I have to focus on just one at a time so that I can make any amount of headway. Otherwise, I fall into the same mental conversations with the same people, having the same heartaches (overthinking is real, my dudes). So, instead of sticking to a rigid schedule that tells me when and how to write... I have a loose schedule that allows for me to write whatever project I am currently inspired to write. It is still necessary, I think, to plan a dedicated time to WORK on what you are writing. Inspiration is delicate AF, so if you keep waiting for it to come around, you'll die with 8 half-written manuscripts on your hard drive. That old cad Hemingway said that, to be a writer, you should not write until you have nothing left to say. Write until you have almost nothing left to say. Then, go for a walk. Get a drink with a friend. Take the kids to the park. Binge watch ALL of Black Mirror. Do something to take your mind off the work. Because, when you go back to it, you will have even more to say.
Granted, writing multiple stories comes at a cost. No matter how many hours a day I write, I am still increasing the completion time of both books by double (more so because I do have a number of non-creative, academic, and otherwise professional pursuits that keep me tap-tap-tappin' dem keys). But, I find that allowing the inspiration that I feel at that time is far more productive than forcing myself into a story that is feeling stale or in which I have expired my inspiration. And, writing until I am almost out of words, stories, emotions, coffee, and steam.
I am still going to tell my students to focus on one thing at a time. And I still try to practice what I preach... to an extent. But that is only because focus and inspiration are not always in sync - so, sometimes, you need to stray from the beaten path to find the best way forward.
Which is far more profound than I had expected it to come across...