I have gotten very good at this over the years. For instance while working on something like one of the Mistborn books, I’d also be writing one of the Alcatraz books. If you read those two, the tones are extremely different. One of the ways I keep things separate is that I generally only write new material for one project at a time. I can edit and revise one project, by taking what it needs to be and making it better, at the same time as I write new material for another project. One of the things you should keep in mind is that when I was writing Wheel of Time books, the struggle was always–even if I wasn’t working on something else at the same time–to make sure that I was remaining true to Robert Jordan’s vision of the characters rather than interpreting them myself. Which means that when it comes time to write a scene from a character’s viewpoint, before I write anything that day I generally read a chapter of Robert Jordan’s work from that character’s viewpoint, and I try to ingrain that in my head and get a resonance going, so that when I sit down to write I can keep the character’s voice straight.
Your question is a little bit like asking an artist, “How can you paint an impressionist painting one day, and then switch to realism the next day?” Well, they’re slightly different arts. Each expresses a painting in its own unique way, and it’s just what you do as an artist. It’s the same difficulty a writer has jumping between characters in a single book. How do I write Shallan in THE WAY OF KINGS and then jump and write Kaladin, and keep them from sounding like one another? It’s something you have to learn to do as a writer. Otherwise, your character voices will all blend together.
Another thing that helps a lot is having assistants. Most authors have to take time out of writing a new novel to stop and approve copyediting and proofreading marks from the last one. I trust my editorial assistant to do most of that, and only come to me with the most important questions. He also compiles all the editorial notes from my various editors, beta readers, etc, so that when I do a revision I only have to look at one document.