I do, more or less.
Typically I focus all my energies on one WIP for a few months. I'll try to take it through one major step (first draft, second draft, revisions, etc.) But I have, at the moment, five or six other ideas on the backburner, at different stages of the project. Even if I'm not directly writing them, my brain is working on the storylines and the revisions. So I guess I'm working on more than one.
Once I'm done with a major step, I let the WIP rest and work on another project.
I don't think I have the ability to stay on one project. My mind needs the different ideas.
"I do not think there is any thrill [...] like that felt by the inventor as he sees some creation of the brain unfolding to success... Such emotions make a man forget food, sleep, friends, love, everything." -- Nikola Tesla