I do this all the time. It's pretty much frowned upon by every productivity expert.

I work on a couple of novels and a couple of short stories at a time.

I've got a book or two or three on the go at once, and pick up the one I fancy reading.

productiivity heresy.

But!

I like it, and I feel like it's a very productive method for me.

Could it be that productivity is not an a one size fits all process?