Right now I'm editing a novel, drafting a novella and planning another novella, and expect some publisher's edits for a short story any day now.
I wouldn't usually be doing the drafting of one and editing another at the same time, but whatever, that story wanted me to write it NOW, so it's happening. I'm getting both done okay, so cool.
The planning of the other one is mostly still in the brainstorming stage, and I give it around 30 minutes a day.
The one thing I try to avoid is having two first drafts on the go at the same time. I did that once and it taught me "never do this again". it messed with my head. I can do it say if I treat one as a side project and work on that only on a specific day for example. But generally I avoid it.
But I do think it's important to be able to switch from one project to another and back again, because that's what I have to do when I'm working on book, but also have one under contract for which publisher's edits could arrive. At that point they're what I have to concentrate on and anything else goes on the back burner. I've learned to cope with that over the last few years. It always makes me chuckle when I see a writer interviewed on TV about their new book, and they're asked about their next book as if they haven't started it yet. Hah! They've probably been working on it for two years already.