I’ve tried to, but I had to keep my focus on one project at a time.
When I get new story ideas percolating inside my head, I do take the time to outline/story-board the idea, so that I can go back to focusing on my one active project.
If I need a day away from my project but still want to write, I will write a bit of creative writing based on those ideas in the wings. This creative writing is not anything that can bloom into a new project.
So, is it possible? Yes. Can I, personally, manage two (or more) projects at once? No.