It's hard, and one or the other is neglected from time to time. The key is not to feel guilty and focus on the one inspiring you at the moment to make the most of that time. I'm in the middle of editing/rewriting one and writing another. Right now, the editing seems to be on the march. :)
I always have more than one project in the works, but I'm not usually actively working on working on more than one at the same time. Recently, though, I did rewrites on one book, while simultaneously proofing the ARC of another -- not just proofing for mistakes, but actively looking for any minor changes that needed to be made, so it was a minor revision, too. I found it so hard, keeping both threads active in my mind, but separate. It really exhausted me. I suspect how well a person handles it depends a lot on that person's modus operandi, mentally speaking. I have a really strong focus, and that's both a plus and minus. When I'm really into something, I hate having to switch gears. Less compulsive people are probably better at it.
I think it depends on how much time your other writing takes up and how depleted you feel afterward. For ex. when I used to work as a reporter and got paid to write more than 40hours a week, well last thing I wanted to do was come home and sit down at the computer. So my personal writing suffered.
Just an additional two cents. Good luck with your writing!
However the way you write, it's got to fit your personality. Lots of writer can work on multiple works. Lots of writers can't. Fit the work to your needs and don't worry what the next guy does. All of us have enough quirks to keep us near a psychologist's couch. Why add more?
Well, the novel's a form of finished writing, but not the only one. I haven't been able to work on two novels at once, but I've definitely written other things while working on a novel. In fact, I'll wager that just from sheer wordcount and revisions, I've written more outside of most novels while writing the novel. I think the differences for me are continuity and complexity. The other writing has been incidental, shorter, with no end-to-end arc or complex character interactions. Just prose, a few ideas, and a normal expository structure. Either that, or a quick short story.
I have started other novels while writing a novel, but usually just enough to give myself a springboard should I ever return to the idea. I personally couldn't keep going on two novels at once. That would take major inspiration and time.
But, I find writing other things while writing a novel definitely keeps the gears lubricated.
I've still got two going and the ending to one was completed, but I still need to bridge a gap in the story. That was making me absolutely crazy, so I had to put it aside. Trying to concentrate solely on the other novel now, and that's working well. I kept the two going for quite some time, but I think it just became too overwhelming for me. Stories I can stick in here and there, but two novels? Way too much self-discipline required, and I'm sorry to say I came up a bit short!
I've also heard that King has a group of writers who do the real 'writing' in his books. He comes up with the idea, the plot structure, then writes the 'fun' parts and leaves the rest to his group of writers. Fact or fiction, who knows.